ASE 8 - Engine Performancefaculty.ccbcmd.edu/~smacadof/Books/A8Student...ASE 8 - Engine Performance Module 1 - Engine Mechanical 1-4 ... A piston inside each cylinder is put into motion

ASE 8 - Engine Performance

Module 1Engine Mechanical

AcknowledgementsGeneral Motors, the IAGMASEP Association Board of Directors, and RaytheonProfessional Services, GM's training partner for GM's Service Technical College wish tothank all of the people who contributed to the GM ASEP/BSEP curriculum developmentproject 2002-3. This project would not have been possible without the tireless efforts ofmany people. We acknowledge:

• The IAGMASEP Association members for agreeing to tackle this large project tocreate the curriculum for the GM ASEP/BSEP schools.

• The IAGMASEP Curriculum team for leading the members to a single vision andimplementation.

• Direct contributors within Raytheon Professional Services for their support oftranslating a good idea into reality. Specifically, we thank:

– Chris Mason and Vince Williams, for their leadership, guidance, and support.– Media and Graphics department under Mary McClain and in particular, Cheryl

Squicciarini, Diana Pajewski, Lesley McCowey, Jeremy Pawelek, & NancyDeSantis.

– For their help on the Engine Performance curriculum volume, Subject MatterExperts, John Beggs and Stephen Scrivner, for their wealth of knowledge.

Finally, we wish to recognize the individual instructors and staffs of the GM ASEP/BSEPColleges for their contribution for reformatting existing General Motors training material,adding critical technical content and the sharing of their expertise in the GM product.Separate committees worked on each of the eight curriculum areas. For the work on thisvolume, we thank the members of the Engine Performance committee:

– Jamie Decato, New Hampshire Community Technical College– Lorenza Dickerson, J. Sargeant Reynolds Community College– Marvin Johnson, Brookhaven College– Jeff Rehkopf, Florida Community College at Jacksonville– David Rodriguez, College of Southern Idaho– Paul Tucker, Brookdale Community College– Kelly Smith, University of Alaska– Ray Winiecki, Oklahoma State University - Okmulgee

ContentsModule 1 – Engine MechanicalAcknowledgements .......................................................................................... 2Module Objectives: ........................................................................................... 4

Introduction ..................................................................................................................... 5Engine Basics ................................................................................................................. 5The Combustion Process ............................................................................................... 6Volumetric Efficiency ..................................................................................................... 10Air Intake Systems ........................................................................................................ 12Forced Induction ........................................................................................................... 15Combustion Chamber Design ....................................................................................... 18Exhaust Systems .......................................................................................................... 19Valvetrain and Valve Timing .......................................................................................... 21Camshaft Timing and Balance Shafts ........................................................................... 22Valve Deposits .............................................................................................................. 23Manifold Vacuum .......................................................................................................... 24Diagnosis ...................................................................................................................... 26Analysis ........................................................................................................................ 26Cylinder Leakage Testing .............................................................................................. 27Running Compression Testing ...................................................................................... 28Module 1 Test................................................................................................................ 31Exercise 1-1 Vacuum Testing ........................................................................................ 33Exercise 1-2 Compression and Cylinder Leakage Testing ............................................ 35Cylinder Leakage .......................................................................................................... 36Running Compression Testing ...................................................................................... 37Exercise 1-3 Exhaust Back Pressure ............................................................................ 38

© 2002 General Motors CorporationAll Rights Reserved

ASE 8 - EnginePerformance

Module 1 - EngineMechanical

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Student WorkbookModule Objectives:Upon completion of this module, the successful learner will be able to:• Explain each of the four cycles of the combustion process.• Describe the effects of barometric pressure and combustion chamber

design on volumetric efficiency.• Identify sources of abnormal combustion

NATEF Area VIII tasks:A7: Perform engine absolute manifold pressure tests; determine

necessary action.A9: Perform cylinder compression tests; determine necessary action.A10: Perform cylinder leakage test; determine necessary action.A12: Perform exhaust system backpressure test; determine necessary

action.




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Student WorkbookIntroductionIn today' s automobile we rely heavily on computers and electronics tocontrol the performance of our engines. For this reason, technicians oftenblame the computer or one of its input/output devices when an engineperformance issue arises. This leads to the unnecessary replacement ofcomponents, a dis-satisfied customer, and a frustrated technician. It isimportant to understand that not even the best computer can compensatefor a worn camshaft, piston rings, intake manifold gasket leak, burnt valve,or any other base engine mechanical problem. Before we look at theintricate electronics, let's first examine the basics.

Engine BasicsThe engine generates power to move the vehicle by burning fuel. Theengine converts the chemical energy of the fuel into mechanical energy(motion). Gasoline is mixed with air and burned in hollow chambers of theengine block, called cylinders. These cylinders are closed at the top by acylinder head. The cylinder head contains valves that open and close toallow airflow in and out of the cylinders. The opening and closing of thevalves must exactly correspond to the up-and-down movement of thepiston in the cylinder for optimum performance. The camshaft and othervalve train components handle this very important job.A piston inside each cylinder is put into motion by expanding gasses whenthe air /fuel charge is ignited. This process is called combustion. Aconnecting rod provides a link between the piston and crankshaft. Themovement of the piston/rod assembly exerts force on the crankshaft andcauses it to rotate. The vehicle is propelled by the collective movements ofthe pistons turning the crankshaft and transmitting this torque through therest of the powertrain.

Figure 1-1




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Student WorkbookThe Combustion ProcessMost automotive engines are four stroke internal combustion engines. Thecombustion process consists of four cycles: 1) intake, 2) compression, 3)power, and 4) exhaust.

The process begins with the Intake stroke, when the air/fuel mixture isdrawn in to the cylinder. The piston moving down in the cylinder from topdead center (TDC) toward bottom dead center (BDC) creates a partialvacuum (negative pressure) in the cylinder. Atmospheric pressure pushesthe air/fuel charge into the cylinder past the open intake valve. Theexhaust valve remains closed during this cycle. The intake valve is closednear the end of the cycle.

The second movement of the piston during the four-stroke cycle is thecompression stroke.During the compression stroke, both the intake and exhaust valves areclosed.The piston moves from the Bottom Dead Center position toward the TopDead Center position. As the piston moves up, the air fuel mixture iscompressed due to the decreasing volume of the cylinder.

Figure 1-2, Cycles of the Combustion Process




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Student WorkbookCompressing the air/fuel mixture raises its pressure, simultaneouslyraising its temperature. This allows the fuel to reach a point close to self-ignition as the piston moves upwards to top dead center. Compressionratios and fuel volatility are closely matched to produce optimum power.

Just before the piston reaches TDC, the spark generated by the ignitionsystem at the spark plug ignites the compressed air/fuel charge. Thisallows the air/fuel mixture to begin burning. The burning of the air/fuelmixture is called combustion. Combustion is started before the pistonreaches Top Dead Center to allow complete combustion of the air/fuelmixture during the power stroke. Combustion creates very hightemperatures in the cylinder. The expanding gases produce high pressure,which forces the piston down. The movement of the piston providesuseable power from the combustion of the air/fuel mixture. This cycle iscalled the power stroke. Each power stroke slightly increases the speed ofthe crankshaft. Since most engines have more than one cylinder, there willbe a slight increase in crankshaft speed during each cylinder's powerstroke. If a cylinder does not provide power during its power stroke, therewill be a noticeable slowing of the crankshaft.

The fourth and final cycle is called the exhaust stroke. Since the cylinderis filled with the exhaust gases created by combustion, these gases mustbe removed from the cylinder before the intake stroke can start again. Asthe piston nears BDC the exhaust valve is opened to begin the exhaustprocess. The exhaust will begin to exit the cylinder even though the pistonis still moving down because the pressure is less in the exhaust manifoldthan it is in the cylinder. The upward movement of the piston continues topush the spent air/fuel charge out through the exhaust port past the openexhaust valve. When the piston reaches near the Top Dead Centerposition, the exhaust valve closes. The engine has completed one fullpower cycle, and the crankshaft has rotated twice. The four-stroke cyclecan now begin again with the intake stroke.

Figure 1-3, Effects of Pressure on Temperature




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Student WorkbookNormal CombustionNormal combustion occurs as the flame front burns across thecombustion chamber from the point of ignition. Normal propagation, orspeed of the flame front, is between 45 and 90 miles per hour. Thisproduces heat, which causes the burned gasses to expand. As thisexpansion occurs, the unburned end gases are compressed even furtherto higher temperatures and pressures. This process creates a great forceon the top of the piston, forcing it downward. However, not all of the heatis transferred into energy. As the hot end gases come in contact with thecylinder head, heat is conducted through the cylinder head to the coolantpassages where it is carried away to the radiator by the coolant. Thepiston also conducts heat to the piston rings, which are in contact with thecylinder walls. This heat is conducted through the cylinder to the coolantpassages in the block where the coolant circulating through the enginecarries it away. The engine oil under the piston also absorbs some of theheat generated by combustion.Another way of controlling heat is turbulence. Since stagnant air can onlydissipate heat at its outer edges, it is important to keep it circulating. If theair/fuel mixture is allowed to become stagnant, the center may becomehot enough to self-ignite, creating abnormal combustion.

Abnormal CombustionThere are typically two types of abnormal combustion; pre-ignition andpost-ignition or spark knock.Pre-ignition occurs when the air/fuel charge self ignites during thecompression stroke before the spark plug fires. Pre-ignition starts from ahot surface in the combustion chamber and causes extremely advancedcombustion. This creates abnormally high cylinder pressures andtemperatures that cannot be sustained, and will quickly lead to severeengine damage.Engine damage resulting from pre-ignition may include melted spark plugelectrodes, melted/scuffed pistons, piston ring damage, and distortedvalve heads.Causes of pre-ignition include "hot" spark plugs (incorrect heat range),excessive accumulation of carbon deposits in the combustion chamber,overheated exhaust valves, and cooling system malfunctions. Pre-ignitionis at its worst at during high speed and load conditions.Spark knock is caused by the flame front initiated by the spark plugcollides with an undesired flame front. The undesired flame front occurswhen part of the unburned air/fuel mixture is compressed to a pressureand temperature that exceeds the "self ignition" point causing it tospontaneously ignite.




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Student WorkbookRemember that when the air/fuel mixture is ignited, the pressure producedtends to compress the gasses that are still unburned. If the heat is notremoved fast enough the mixture can self ignite. As the flame frontscollide an audible knock can be heard. Spark knock generally occursduring conditions of heavy load at low to medium engine speeds.Common causes of spark knock include low octane fuel, excessive carbondeposits, EGR malfunctions, high combustion chamber temperature, overadvanced ignition timing, and high compression ratios. Spark knockcauses a rapid rise in temperature and pressure that can damage sparkplug electrodes, pistons, rings, valves, and valve seats.

Figure 1-4, Pre-Ignition Figure 1-5, Post-Ignition

Note:Two terms that you should be familiar with are quenching and turbulence.Quenching occurs as the heat generated by the compression of the air/fuel mixture between the cylinder head and the piston is transferred to thecylinder head and cooling jackets. This heat transfer helps to controlcombustion chamber temperature and prevent abnormal combustion.Turbulence refers to the swirling or tumbling of the combustion mixturethat improves air/fuel mixing and helps to control combustion chambertemperature.




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Student WorkbookVolumetric EfficiencyWhen the piston is at Bottom Dead Center the space above the piston isthe volume of that cylinder. This volume is the maximum amount of air/fuelmixture that can be placed within the cylinder. The volume of a cylinder isdivided between the piston displacement and the combustion chamber.Piston displacement contains the volume from bottom dead center to topdead center. The combustion chamber is the volume above the piston attop dead center. The volume of a cylinder is measured in cubiccentimeters or liters. The difference between cylinder displacement atBDC and TDC is called compression ratio.

Note:Engine displacement is the sum of each cylinder's piston displacement.Engine displacement does not account for the volume of the cylinderhead.

A common way to describe an engine is that it is just a big air pump. Theability of an engine to draw in air under various running conditions isreferred to as its volumetric efficiency rating. An engine that cancompletely fill the cylinders with a fresh air/fuel charge during the intakestroke is said to have high volumetric efficiency. Since only about 25% ofthe energy produced during combustion is used to propel the vehicle, asmall change its ability to "breathe" will have a very large impact on engineperformance. The actual volume of air/fuel mixture that can enter acylinder is usually less than the maximum volume of the cylinder.Volumetric efficiency is expressed in a percentage. If a cylinder has avolumetric efficiency of 80 percent, the actual volume is 20 percent lessthan the maximum volume. The actual volume is determined by acombination of outside conditions, engine design and engine speed.

Figure 1-6, Volumetric Efficiency




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Student WorkbookConditions Outside of the EngineSince oxygen is needed for burning fuel, the amount of oxygendetermines how much fuel can be burned during combustion, regardlessof the quantity of fuel. Air temperature, atmospheric pressure andatmospheric conditions control how much oxygen is in a cubic centimeterof air. This is called air density. High temperatures result in less oxygen ina cubic centimeter of air. Low temperatures result in more oxygen in acubic centimeter of air.Since a cylinder can only hold a specific volume of air, air temperature willlimit the amount of oxygen in that volume and the amount of combustionthat can occur. The amount of combustion determines the power output.The affect of atmospheric pressure on air density is slightly different thanair temperature. When atmospheric pressure is high there is more oxygenper cubic centimeter of air. When atmospheric pressure is low there isless oxygen per cubic centimeter of air. Atmospheric pressure changesbased on altitude. At sea level the average atmospheric pressure is 14.7pounds per square inch and decreases as the altitude increases. In anengine's cylinder, high atmospheric pressure results in an air/fuel mixturewith more oxygen. This produces more efficient combustion and higherpower output. When the atmospheric pressure is low, the power output islower. An engine that "breathes" at atmospheric pressure is referred to asa naturally aspirated engine.

Note:Engine horsepower of a naturally aspirated engine is reduced by about3% per one thousand feet above sea level. For example, an engine thatproduces 150 bhp at sea level will only produce 85 bhp at the top of Pike'sPeak Colorado (14,110 ft.).

Humidity is a part of atmospheric condition that can also affect the amountof oxygen in a cubic centimeter of air. When humidity is high, there is anincrease of water vapor in the air. The water vapor displaces oxygen. Withless oxygen per cubic centimeter, high humidity will reduce combustionefficiency and reduce power output of the engine. We are not the onlyones that have a hard time breathing on hot humid days.

Engine Speed and Its Affect on Volumetric EfficiencySince flow is based on time, engine speed will determine the amount oftime for both air/fuel mixture and exhaust gas flow. As the engine speedincreases less time is allowed for flow through the engine. Volumetricefficiency typically decreases at high engine speeds since there is lesstime for air to fill the cylinders.




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Student WorkbookEngine Design Factors for Volumetric EfficiencyAlthough air temperature and atmospheric pressure determines theamount of available oxygen per cubic centimeter, the engine design andengine speed determines the volume of air/fuel mixture that can bedelivered to the cylinder. The design of the intake system, combustionchamber and the exhaust system all determine the flow of the air/fuelmixture.

Air Intake SystemsBefore outside air can enter the engine cylinder it must flow through thecomponents of the intake system. The intake system limits the air/fuelmixture available to the combustion chamber. The intake system can onlyallow a limited amount of air/fuel mixture per second to the combustionchamber. Induction systems are matched, or tuned, to a particularapplication. Air filter surface area, throttle body bore diameter, and eventhe tubing connecting the air filter housing to the throttle body control theair charge to the engine. Once the air enters the throttle body it is theintake manifold's job to bring it the rest of the way to the combustionchamber when the intake valve opens.

On engines equipped with TBI, the intake manifold must carry both air andatomized fuel to the combustion chamber. These manifolds must bedesigned with compromises to meet the needs of both air and fueldelivery. "Wet" manifolds, as they are known, must maintain enoughvelocity throughout the desired operating range to hold fuel in suspensionwhile maintaining sufficient air capacity to obtain peak horsepower. If airvelocity is not great enough the fuel will fall out of the air and condense ofthe manifold walls, causing a lean condition.Intake manifolds for Port Fuel Injection, PFI, systems do not have to carryfuel and can be tuned for either maximum torque or horsepower. Longintake runners, for increased low-end and mid-range torque, are possiblewithout the concern of fuel condensing on the manifold walls.

Figure 1-7, TBI Airflow




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Recent changes in manifold design have helped to improve volumetricefficiency. Many of today's manifolds are larger and have bigger runnersto handle an increased volume of air.The use of thermoplastic manifolds instead of cast aluminum is also agrowing trend. Manifolds made from thermoplastics are lighter, dissipateheat better, have smoother internal surfaces, and are much cheaper tomanufacture than their aluminum counterparts. An Over-PressurizationRelief valve may be used to reduce the chance of damage if combustionpressures enter the intake manifold.The more turbulent, or erratic the air is as it moves through the manifold,the greater velocity it will have moving into the combustion chamber whenthe intake valve opens. This helps to atomize the fuel and fill the cylindermore completely on the intake stroke. Combined with the propercombustion chamber design, turbulence also helps to reduce combustionchamber temperatures as the air/fuel mixture burns, reducing NOxemissions.The air intake system determines the engines torque curve. A longerintake manifold results in higher torque at lower RPM, and a shorter intakemanifold results in higher torque at higher RPM. Some intake designs canchange the way air flows through them at different RPM's to optimizetorque output.One such manifold design can be found on the Cadillac Catera with the3.0L (L81) and CTS with the 2.6L (LY9) and the 3.2 (LA3) engines. Theintake manifolds on these engines are fitted with 2 valves that can be setfor 4 different intake manifold lengths. The 4 different manifold lengthsobtainable result in different torque curves with maximum torque atdifferent engine RPM. The intake plenum switchover valve is located inthe intake manifold between the cylinder heads. The intake resonanceswitchover valve is located between the 2 resonating pipes connected tothe intake manifold. The control module controls the 2 valves by means ofsolenoid valves and vacuum operated diaphragm units.

Figure 1-8, AFI Airflow




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Figure 1-9, Resonator Valve and Related Parts 3.0L (L81)

(1) Air Intake Duct (5) Bracket Mounting Bolts(2) Retaining Clip (6) Bracket(3) Intake Resonator (7) Resonator Valve Actuator(4) Vaccum Hose

Figure 1-10, Plenum Switch-Over Valve, Actuator and Solenoid




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Student WorkbookAt idle and engine speeds up to 3200 RPM, the resonance valve ispositioned for air flow to continue through the separate passages of theresonance valve assembly. The air flows from the resonance valveassembly through the throttle body ducts to the throttle body. At idle, airflows through the idle air passage or past the throttle plates into the intakemanifold. The plenum switch-over valve is positioned so that idle airflow isdistributed equally between all of the cylinders. As the engine speedincreases to 3200 rpm, the plenum switch-over valve moves to separatethe two halves of the intake manifold. The long airflow path provideshigher engine torque at low rpm.

When the engine speed is between 3200 and 4100 RPM, the resonanceswitch-over valve is positioned to connect the two passages of theresonance valve assembly. The active length of the air intake isshortened. This provides the airflow necessary for maximum torque in thisrpm range. At 4100 RPM the plenum switch-over valve is positioned toconnect the two sides of the intake manifold. This provides the shortestactive air intake length and provides the correct amount of airflow formaximum torque at this engine rpm range.

Forced InductionTo get more power from an engine, more air and more fuel must moveinto the combustion chamber during the intake stroke. Engines with forcedair induction have intake manifold pressure values that vary from belowatmospheric pressure, or a vacuum, to higher than atmospheric pressure,or boost pressure. To overcome the limitations of natural aspirationturbochargers and superchargers act as a pump to increase air intakepressure and the amount of air that goes into the cylinders.

Figure 1-11, Muti-Ram System Operation




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Student WorkbookTurbochargersThe turbocharger is mounted between the air cleaner assembly andthrottle plates. It operates as a centrifugal pump to increase the pressurein the air intake and move more air into the cylinders. Exhaust gases aredirected to the turbocharger. The flow of the exhaust gases turns theturbine on the exhaust side of the turbocharger. The compressor turnswith the turbine. Air enters the compressor from the air cleaner assembly.The rotating force of the compressor boosts the pressure of the intake air.At a higher pressure, the air is denser and contains more oxygenmolecules. With an increase in air density, more fuel can be added to themixture. Forcing more air and fuel into the cylinder increases thevolumetric efficiency. During combustion, more power is produced to pushthe piston on the power stroke.

The rotational force of the compressor varies with exhaust flow. When theengine speed is low or under light load, the exhaust flow turns theturbocharger at a slower speed. The boost pressure is low at this time. Asthe throttle is opened, the increased engine load and speed creates moreexhaust flow. The increased exhaust flow causes an increase in boostpressure. The turbocharger has the ability to increase boost pressure highenough to cause detonation and engine damage. A wastegate allowssome exhaust gases to bypass the turbine. The wastegate opens to limitthe boost pressure to a predetermined amount.

Figure 1-12, Turbocharger Operation




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Student WorkbookNote:A turbocharger does not provide boost when the engine is at idle or duringlight load. When the engine load increases, the turbocharger speed mustincrease before boost pressure is available. The time that elapsesbetween when the load increases and the boost increases is referred toas "turbo-lag".

Because the wheel/shaft may spin at speeds in excess of 100,000 rpm,the bearings are cooled and lubricated with engine oil. Constantlubrication of the turbocharger is important to avoid failure. To help coolthe turbocharger bearings and prevent damage, you should allow theengine to idle before shutting it off.

SuperchargersA supercharger is a positive displacement pump. Some 3800 engines usea supercharger that is mounted to the intake manifold and driven with abelt from the engine crankshaft. With the engine running, the rotors rotatein opposite directions. Intake air is compressed as it moves through thesupercharger housing. Compressing the air boosts its pressure. At ahigher pressure, air is denser and contains more oxygen molecules. Morefuel can be added to the air. During combustion more power is producedto push the piston on the power stroke.

Figure 1-13, Supercharger Operation




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Student WorkbookBecause the supercharger is connected directly to the engine crankshaft,boost pressure is available at low engine rpm as well as through the wholerange of engine speeds. As a positive displacement pump thesupercharger does have a maximum boost pressure that it can produce.To limit boost pressure, a bypass valve operates to recirculate boostpressure to the supercharger inlet. Limiting boost reduces the amount ofdrag put on the engine by the supercharger, as well as maximizes fueleconomy.The supercharger used on the 3800 engine is a Roots type. Two rotorswith three lobes each have a helical twist to quiet the operation andsmooth air pulses. The rotors run at a minimal clearance to each otherand the supercharger housing. The rotors are timed to each other with apair of gears. The gears are turned with the input shaft, which is driven bythe engine drive belt. The rotors spin at approximately two times theengine's rpm. Since the supercharger is driven off the crankshaft, boost isalways available. There isn't any "lag" time to get the compressor up tospeed as there is on a turbocharger. However, the overall gain involumetric efficiency is slightly overshadowed by the power it robs fromthe engine to turn the gears of the supercharger.

Combustion Chamber DesignThe combustion chamber limits the flow of the air/fuel mixture into thecylinder. The size of the intake valve or valves and the design of thecombustion chamber will only allow a limited amount of flow into thecylinder per second. There are many different types of combustionchambers. Each design has specific advantages and disadvantages.However, the ultimate goal of any combustion chamber design is tomaximize volumetric efficiency and create conditions within the cylinderthat allow complete combustion.The hemispherical design combustion chamber uses one intake valve andone exhaust valve. The combustion chamber has a half ball shape withthe spark plug located between the valves. This design provides very goodairflow, but offers very little turbulence of the air/fuel charge entering thecylinder. Under typical driving conditions combustion often suffersbecause of the large volume of the combustion chamber. This designrequires more time for the flame to completely burn the fuel and generallyresults in higher emissions.




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Figure 1-14, Combustion Chamber Design

GM uses a modified version of the hemispherical design in its four-valveper cylinder applications, known as the pentroof design. This designdiffers from the hemispherical design in that the two intake and twoexhaust valves are seated on a plane with the spark plug in the center.This creates a flat "pentroof" shape combustion chamber instead ofspherical shape. This four-valve design combines improved airflow andturbulence to provide greater volumetric efficiency at high RPM.As with the pentroof design the wedge design is named after the shape increates. This design works well for a two-valve system. The wedge shapedesign offers slightly lower volumetric efficiency at high RPM. However, itprovides increased air/fuel turbulence for better mixing and more completeburning. This results in better performance at lower RPM.

Note:The GM "Small Block" engine family is well known for this type of cylinderhead.

Exhaust SystemsThe exhaust system has a major affect on the air flowing through theengine. The exhaust must be the correct size to allow the engine to expelthe exhaust gases at the proper rate. It must allow for good flow withminimum backpressure. Backpressure is created when exhaust gasesback up in the exhaust system because they can't be expelled as fast asthey are produced. If exhaust gases are not expelled effectively, the air/fuel charge to the cylinder will be diluted and the velocity of the in comingintake charge will be drastically reduced. The resulting performance will beless than satisfactory. If the diameter of the exhaust pipes is too big, theengine may run cooler, resulting in increased emissions levels.




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Student WorkbookAlthough the operation is simple, the design of the exhaust system is oftenmore complex. High-pressure pulses produced by the exhaust valvesopening and closing must be compensated for to reduce exhaust noise.This is accomplished through manifold design and the muffler. Exhaustmanifolds must fit in and around cramped engine compartments, leaveadequate space for serviceability, and contain no sharp bends that couldrestrict flow.

A major component in the exhaust system is the catalytic converter. Itsprimary function is to chemically convert harmful exhaust gases such ashydrocarbons, carbon monoxide, and NOx into more environmentallyfriendly water, carbon dioxide, and nitrogen. Deterioration of the materialsinside the converter, possibly due to a sustained misfire condition or richmixture, can cause the exhaust to become restricted.

If an Exhaust restriction occurs, the customer concern will most likely belack of power. Follow service manual procedures to check for a restriction.Although specifications vary between vehicle platforms, typically thereshould be less than 1.25 psi of backpressure at 2,500 RPM with theengine at normal operating temperature with no load on the engine.

Figure 1-15, Exhaust System




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Student WorkbookValvetrain and Valve TimingThe final area in engine design that affects volumetric efficiency is thevalvetrain. Valvetrains come in various configurations including push rod,single and dual over-head overhead camshaft designs. No matter whichdesign is used, proper camshaft timing, valve lift, and the duration of liftare necessary for good engine performance and emissions control.The camshaft is responsible for controlling the rate of valve opening andclosing. It also determines the length of time that the valve is open. Thesefunctions are identified through the terms lift, duration, and overlap.

LIFT is the distance that the valve is moved off of its seat when fully open.Valve lift is generally measured at the camshaft lobe, but can also bemeasured at the valve.DURATION is the length of time that the valve remains open. Since theactual time in seconds varies with engine speed, duration is measured indegrees of crankshaft rotation. Ideally, the duration event begins beforethe piston changes direction during a stroke.OVERLAP is the amount of time (in crankshaft degrees) that both theintake and exhaust valves are open at the same time. As we learnedearlier, the intake stroke begins before the piston reaches TDC of theexhaust stroke. Since the exhaust is hot and under pressure, the inertia ofthe gasses actually causes their flow to continue after the piston passesTDC. Just before TDC (about 12 ) the intake valve opens. The air/fuelmixture begins to fill the cylinder even though the piston is still movingupward. This is because the inertia created by the exiting exhaust gasescreate a siphoning or venture effect increasing volumetric efficiency. Thisis called scavenging.

A worn camshaft lobe will cause the valve to open later, close sooner, andreduce valve lift. Engine performance will suffer due to the loss involumetric efficiency.

Figure 1-16, Valve Timing Figure 1-17, Camshaft Lobe




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Student WorkbookCamshaft Timing and Balance ShaftsIn order for the engine to perform correctly, the camshaft must besynchronized to the movements of the crankshaft. This will ensure that thevalves open at the correct time relative to piston position. The camshaft(s)is/are driven by a chain or belt at one-half crankshaft speed. Aligning thetiming marks on the camshaft and crankshaft sprockets to their correctposition will set static timing. Static timing provides a baseline relationshipbetween the crankshaft and camshaft(s). A worn timing chain or sprocketswill cause the camshaft to lag behind crankshaft rotation changing whenthe valves open and close in reference to piston position. This will reducethe volumetric efficiency and power output of the engine.

Figure 1-18, Static Timing Marks 4.2L (LL8)




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Student WorkbookSome engines have a natural dynamic engine imbalance due to the firingpulses and the forces exerted on the crankshaft. This can cause customerconcerns of "running rough" or "missing" when no problem actually exists.Engines that have a strong natural imbalance may contain one or morebalance shafts to minimize vibration levels. These balance shafts must beproperly timed to the crankshaft to counteract the deflections producedduring rotation. One such engine that utilizes this feature is the 2.2L (L61)Ecotec engine used in J and N body passenger cars. It is important for thetechnician to be able to differentiate between normal engine imbalanceand abnormal imbalance such as a misfire to prevent misdiagnosis.

Valve DepositsCarbon deposits on the valves can cause drivability concerns such as coldstumbles, hesitation, and stalling. This carbon absorbs the fuel intendedfor combustion in the cylinder causing a lean condition. As the depositgrows, fuel will become further restricted causing increased combustionchamber temperatures and higher NOx emissions. Excessive valvedeposits can be caused by oil, fuel, excessive heat, and PCV systemmalfunctions.

Figure 1-19, Balance Shafts w/Chain (2.2 L61)




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Student WorkbookManifold VacuumThe intake stroke of the piston creates a vacuum in the manifold. Vacuumis any pressure lower than atmospheric pressure. The greater thedifference between the low pressure in the cylinder and high pressure inthe atmosphere, the better the distribution of air and fuel to the cylinders.An engine's ability to form and hold a vacuum is directly related to itsability to form and hold compression. When an engine loses the ability tocreate vacuum, performance suffers.The amount of vacuum formed in the manifold depends on several things.First, the cylinders must be sealed. If a cylinder has low compression orhigh leakage, it will not produce sufficient vacuum to draw in the air/fuelmixture. If the manifold is not sealed, vacuum will be lower than normal.Vacuum hoses, vacuum operated systems, and accessories that operateon vacuum may also leak, causing lower manifold vacuum. In themanifold, when the throttle plate is closed at idle, the vacuum is greater.When the throttle plate is open and atmospheric pressure enters themanifold, vacuum is lower.Monitoring vacuum is a quick and easy way to test an engine. It is a goodindicator of the engine's ability to run efficiently. Typical engine vacuum isa steady reading between 15 and 22 inches Hg with the engine at normaloperating temperatures, idle, and in drive. Vacuum changes with load, sooperating accessories when monitoring vacuum will change the readings.Vacuum readings will vary between engines. One reason may bedifferences in the compression ratio. If the engine has highercompression, it will have 1 to 2 inches Hg higher vacuum. Altitude alsoaffects vacuum. For every 1,000 feet above sea level, vacuum will belower by 1 inch Hg. Some engines use a high lift cam or haveconsiderable valve overlap. Thiswill produce a slightly lower, erraticneedle reading on the gauge.Some things that can bediagnosed using vacuumreadings are enginecomponents (i.e., valves,valve guides and springs,piston rings), manifold leaks,timing, and a restricted exhaustsystem. In addition, a lowermanifold vacuum can also

Figure 1-20, Manifold Vacuum




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Student Workbook




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Student WorkbookDiagnosisCranking "static" Compression TestingAs we have learned, compression is a critical component of combustion. Ifa combustion chamber is not properly sealed there will be a loss ofcompression resulting in a loss of performance. When an enginemechanical issue is suspected or a vacuum test indicates a problem, acranking compression test should be performed.1. Charge the battery if the battery is not fully charged.2. Disable the ignition system.3. Disable the fuel injection system.4. Remove all the spark plugs.5. Block the throttle plate wide open.6. Start with the compression gauge at zero and crank the engine through

four compression strokes (four puffs).7. Make the compression check for each cylinder. Record the reading.8. If a cylinder has low compression, inject approximately 15 ml (one

tablespoon) of engine oil into the combustion chamber through thespark plug hole. Recheck the compression and record the reading.This is called a "wet" compression test.

Analysis• The minimum compression in any one cylinder should not be less than

70 percent of the highest cylinder.• No cylinder should read less than 690 kPa (100 psi). For example, if

the highest pressure in any one cylinder is 1035 kPa (150 psi), thelowest allowable pressure for any other cylinder would be 725 kPa(105 psi). (1035 x 70% = 725) (150 x 70% = 105).

• Normal - Compression builds up quickly and evenly to the specifiedcompression for each cylinder.

• Piston Rings Leaking - Compression is low on the first stroke.Compression then builds up with the following strokes but does notreach normal. Compression improves considerably when you add oil.

• Valves Leaking - Compression is low on the first stroke. Compressionusually does not build up on the following strokes. Compression doesnot improve much when you add oil.

• If two adjacent cylinders have lower than normal, suspect leaking headgasket




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Student WorkbookCylinder Leakage TestingWhen a cranking compression test indicates that a compression leak ispresent, it is necessary to determine its source before enginedisassembly. The more you know about the area of the fault, the easierand less time consuming it will be to correct it. By introducing compressedair into the cylinder and monitoring the amount of pressure loss using the -J 35667-A Cylinder Leak down Tester (or equivalent) and listening forescaping air the exact location of the compression loss can bedetermined.

1. Disconnect the negative battery cable.2. Remove the spark plugs. Refer to Spark Plug Replacement in Engine

Controls.3. Install the J 35667-A.4. Measure each cylinder on the compression stroke, with both valves

closed.Note: Hold the crankshaft balancer bolt in order to prevent pistonmovement.5. Apply air pressure, using the J 35667-A. Refer to the manufacturer's

instructions.6. Record the cylinder leakage readings for each cylinder.Note:Normal cylinder leakage is from 12 to 18 percent. Make a note ofany cylinder with more leakage than the other cylinders. Any cylinder with30 percent leakage or more requires service.7. Inspect the 4 primary areas in order to properly diagnose a leaking

cylinder.8. If air is heard from the intake or exhaust system, perform the following

procedure: Remove the valve rocker arm cover of the suspect cylinderhead. Ensure that both valves are closed. Inspect the cylinder head fora broken valve spring. Remove and inspect the suspect cylinder head.Refer to Cylinder Head Cleaning and Inspection.

Figure 1-21, Cylinder Leakage Tester




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Student Workbook9. If air is heard from the crankcase system at the crankcase (oil fillertube), perform the following procedure:Remove the piston from the suspect cylinder. Inspect the piston andconnecting rod assembly. Refer to Piston, Connecting Rod, andBearings Cleaning and Inspection. Inspect the engine block. Refer toEngine Block Cleaning and Inspection.

10.If bubbles are found in the radiator, perform the following procedure:Remove and inspect both cylinder heads. Refer to Cylinder HeadCleaning and Inspection. Inspect the engine block. Refer to EngineBlock Cleaning and Inspection.

11. Remove the J 35667-A.12.Install the spark plugs. Refer to Spark Plug Replacement in Engine

Controls.13.Connect the negative battery cable. Refer to Battery Negative Cable

Disconnect/Connect Procedure in Engine Electrical.

Running Compression TestingThe typical “static” cranking compression test does a good job of checkingoverall cylinder seal, and will identify gross compression pressure leaks.However, we have learned that volumetric efficiency is highest at slowengine speeds because the piston moves slower and allows more time forthe air/fuel charge to enter and the exhaust to leave the cylinder.In order to determine the overall “breathing” ability we must measure theactual amount of air moving through the engine. We do this by checkingcompression produced with the engine running.Start with a normal (“static”) compression test. To eliminate rings, valves,holes in pistons, that sort of thing. A normal cylinder balance test is alsohelpful (so you know which, if any, cylinder is presenting a problem).Engine should be warm.Put all spark plugs but one back in. Ground that plug wire to preventmodule damage.Disconnect that injector on a port fuel system.

Caution:Before servicing any electrical component, the ignition key

must be in the OFF or LOCK position and all electrical loadsmust be OFF, unless instructed otherwise in these procedures.

A tool or equipment could easily come in contact with a liveexposed electrical terminal; also disconnect the negative

battery cable. Failure to follow these precautions may causepersonal injury and/or damage to the vehicle or its components.




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Student WorkbookPut your compression tester into the empty hole. The test can be donewithout a Schrader Valve, but most people recommended leaving thevalve in the gauge and “burping” the gauge every 5-6 “puffs.”Start the engine and take a reading. Write it down.Now goose the throttle for a “snap acceleration” reading. Reading shouldrise. Write it down.

Note: Don’t use the gas pedal for this snap acceleration. The idea is tomanually open then close the throttle as fast as possible without speedingup the engine. This forces the engine to take a “gulp” of air.

Now, write down your readings for at least the bad cylinder (if there is asingle bad cylinder) and maybe 2-3 good ones.Make a chart like this:

CYL STATIC COMPR IDLE-RUNNING COMPR SNAPCyl 1 150 75 125Cyl 2 175 80 130Cyl 3 160 75 120Cyl 4 160 80 125

Analysis:Running compression at idle should be 50-75 psi (about half crankingcompression). Snap throttle compression should be about 80% ofcranking compression.

Sample 1 - Restricted ExhaustCYL STATIC COMPR IDLE - RUNNING COMPR SNAPCyl 1 150 75 180

If snap measurements are higher than 80% of cranking measurements,look for restricted exhaust on that cylinder - such as worn exhaust camlobe, or collapsed lifter. Or, if these are all high, look for a clogged catalyticconverter or collapsed exhaust pipe.




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Student WorkbookSample 2 – Restricted IntakeCYL STATIC COMPR IDLE-RUNNING COMP SNAPCyl 1 150 60 90

If snap measurements are less than 80% of cranking measurements, lookfor restrictions in the intake side of that cylinder – such as a worn intakecam lobe, collapsed lifter, bent pushrod, or excessive carbon build up onthe valves. If all measurements are low, look for a dirty air filter orcollapsed air inlet hoses.




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Student WorkbookModule 1 Test1. A manifold vacuum test should be used to check the condition of the:

a. ignition systemb. fuel systemc. base engined. cooling system

2. In addition to valve and head gasket concerns, irregular, low, orfluctuation manifold vacuum test readings may be the result of:a. plugged PCV valveb. late ignition timingc. excessive compressiond. exhaust leak

3. Which of the following will cause excess exhaust back pressure?a. Exhaust valveb. Defective HO2S sensorc. Plugged catalytic converterd. Worn valve guides

4. Excessive exhaust back pressure is most noticeable during_________speed, _______load conditions.a. low, lowb. low, highc. high, highd. high, low

5. Why is spark knock a potential result of too little EGR?a. Excessive exhaust manifold/catalyst temperatureb. Excessive combustion chamber temperaturec. Air/fuel ratio too richd. Reduced emission levels




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Student Workbook6. What occurs when the air-fuel mixture self ignites during thecompression stroke before the spark plug fires?a. Spark knockb. Misfirec. Pre-ignitiond. Valve slap

7. Short intake runners are desirable for peak performance at__________ RPM.a. Lowb. Highc. 1000d. mid-range

8. Which of the following is NOT a benefit of recent changes in intakemanifold design?.a. Lighterb. Strongerc. Cheaperd. Increased air capacity

9. Technician A says atmospheric pressure decreases as altitude isincreased. Technician B says that atmospheric pressure at sea level isabout 14.7 lbs. per square inch. Who is Correct?a. Technician Ab. Technician Bc. Bothd. Neither

10.Humid air has more oxygen molecules per cubic centimeter than dryair.a. Trueb. False




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Student WorkbookExercise 1-1 Vacuum TestingObjectives: Following completion of this worksheet, you will be able to:• Perform a manifold vacuum test• Use your knowledge and critical thinking skills to identify possible

engine performance problems associated with engine mechanicalcondition based on test results

Locate an unrestricted intake vacuum port and connect a vacuum gaugewith three feet of hose to it. Try to find a source as close to the manifoldas possible. Start the engine and let it reach normal operating temp.

1. With the engine idling in drive (neutral for M/T), with all accessories off,and the parking brake set for safety, observe the gauge. What is thereading on the vacuum gauge? __________________

2. Does the reading fall within an acceptable range and remain steady?_____________

3. If the vacuum gauge needle fluctuates or drops 1 to 2 inches Hg lowerthan normal at regular intervals what could be the cause?______________________________________________________________________________________________________

4. What would cause a steady reading that was lower than normal (12-15" Hg)? ____________________________________________________________________________

Place the transmission in park and quickly open then close “snap” thethrottle.

5. What does the vacuum gauge read at wide open throttle?______________________

6. What caused this change in vacuum?______________________________________

7. Observe the vacuum reading just after snapping the throttle closed.What is the reading on the gauge? _____________ What is this anindication of? _________________________________________________________________________________

8. If the vacuum did not reach 22-24 inches Hg when the throttle issnapped shut, what would be the next logical diagnostic test toperform? ___________________________




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Student WorkbookGradually increase engine speed to 2000 RPM, hold it there for one fullminute.9. What is the vacuum reading at 2000 RPM?

________________________

10.Earlier we saw the vacuum drop off completely when the throttle wasfully opened. Why did that not occur as the throttle was opened to2000 RPM? ________________________________________________________________________________________________________________________________________________________________________________________________________________________________

11. What would be indicated if the vacuum gauge read less than 5 inchesHg at 2000 RPM?_______________________________________________________________

12.If the reading was as indicated above, what test would you performnext?_____________________________________________________________________

Stop the engine and disable the fuel and ignition systems. Restrict theairflow into the intake by using the IAC valve tester to bottom out the IACvalve in its bore or by covering the throttle body with your hand. Observethe reading on the vacuum gauge while cranking the engine. This is calleda cranking vacuum test. This test can provide a good indication of overallcylinder seal. The gauge reading should be above 2.5 inches Hg.

13.What is the vacuum reading on the gauge during cranking?_____________________

14.What does each fluctuation of the needle on the gauge indicate?___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

15.Is the gauge reading above 2.5 inches Hg? ___________________




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Student WorkbookExercise 1-2 Compression and Cylinder Leakage TestingObjectives: Following completion of this worksheet, you will be able to:• Perform a cranking compression test (wet and dry)• Perform a cylinder leakage test• Perform a running compression test• Use acquired knowledge and critical thinking skills to identify

performance problems based on the results of the compression andleakage tests

Use Service Information to locate the procedure for compression testingon the chosen ASEP vehicle.

1. List four steps necessary when preparing for a cranking compressiontest._________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

2. Install the compression gauge into #1 spark plug hole and crank theengine through four compression strokes “puffs”. Observe the readingon the first puff, then after four. Record your results below. Repeat forthe remaining cylinders.

3. What is the minimum allowable compression according to the serviceinformation? __________________________________

4. The first “puff” should be within ________% of the final reading.

5. All cylinders should be within what percentage of the highest cylinder?____________

6. Were all of the cylinders within the specified range of each other?________________

Cylinder 1st Puff After 4 Cylinder 1st Puff After 4 #1 #5 #2 #6 #3 #7 #4 #8




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Student Workbook6. What would be the next step in your diagnosis if any cylinders werelower than specified?____________________________________________________________

7. If compression increases after performing the test in the abovequestion, what is the most likely cause of the low compressionreading? ____________________________

8. If cylinder compression remains abnormally low, what test should beperformed to isolate the cause of the poor cylinder seal?__________________________________

Cylinder LeakageRotate the crankshaft so the #1 piston is at TDC compression stroke.Calibrate and install the J-35667-A, or equivalent, apply air to the cylinder.To ensure accuracy the engine should be warm.

1. Record your leakage readings below:

2. No cylinder should have more than _____% leakage.

3. List three possible causes for high leakage and air bubbles present inthe radiator or surge tank.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

4. List the most likely cause of air escaping at the throttle body.________________________________________________________________________________________

5. While performing a cylinder leakage test you measure 18% leakageand hear escaping at the oil fill. What is indicated?___________________________________

6. What is the correct spark plug torque for this engine?_________________________

Cylinder Reading Cylinder Reading #1 #5 #2 #6 #3 #7 #4 #8




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Student WorkbookRunning Compression TestingReinstall all of the spark plugs, but one. Disable the ignition anddisconnect the fuel injector on PFI engines if possible for the remainingcylinder. Install the compression gauge and start the engine. Observe thecompression gauge at idle and snap throttle. Remember to relieve thepressure every 5-6 puff if the Schrader valve was not removed from thehose.

1. Record your results below:

2. “Snap” throttle compression should be _______% of crankingcompression.

3. Why is idle compression lower than cranking compression?_________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

4. List three causes for lower than normal “snap” throttle compressionreadings.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

5. List three causes for higher than normal “snap” throttle compressionreadings.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

6. Running compression is a good way to measure an engine’s________________________________________________________________________________________.

Cylinder Cranking Compression (from

previous test)

Idle Compression

“Snap” Throttle

Compression #1 #2 #3 #4 #5 #6 #7 #8




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Student WorkbookExercise 1-3 Exhaust Back PressureObjectives: At completion of this worksheet, you will be able to:• Perform an exhaust system back pressure test• Use acquired knowledge and critical thinking skills to determine

possible engine performance problems based on the results of theback pressure test

1. What symptom or complaint would most likely be associated with arestricted exhaust?_____________________________________________________________________

2. Use Service Information to locate the exhaust system restriction testprocedure for the chosen ASEP vehicle. What is the documentidentification number for this procedure? __________________

3. Perform the test as described in Service Information and record yourresults.At idle ____________psi At _______(specified) RPM________psi

4. What is the maximum specification for back pressure?___________psi

5. Does this vehicle exhibit excessive exhaust restriction? __________

6. List three possible causes for exhaust system restriction._____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

7. If you determine that the restriction is due to a failed catalytic converterit is very important to determine the root cause of the failure beforereturning the vehicle to service. List two possible causes for converterfailure._____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Note: Some vehicles incorporate more than one catalytic converter in theexhaust system. For instance, some trucks have small converters in the"Y" pipe for each bank of cylinders. Make certain to test backpressure onboth banks if exhaust system restriction is suspected.

ASE 8 - Engine Performancefaculty.ccbcmd.edu/~smacadof/Books/A8Student...ASE 8 - Engine Performance Module 1 - Engine Mechanical 1-4 ... A piston inside each cylinder is put into motion

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