22 A8 — Engine Performance Practice Questions & Answers

1. 1. A driver reports an intermittent stumble that happens about once a week. The vehicle runs normally in the shop and no confirmed trouble codes are stored. What is the best first step?

   • A.Run a cylinder leakage test on every cylinder before looking at any electronic data
   • B.Clear all stored data first so that the next occurrence is captured in a clean memory
   • C.Replace the sensors most often blamed for a stumble and ask the driver to report back
   • D.Interview the driver about the conditions and check pending codes and freeze frame data✓ Answer

   The first task in any driveability diagnosis is to verify the complaint and learn the exact conditions that produce it, then preserve the evidence already in the controller. Pending codes and freeze frame record what the engine was doing when the fault occurred, and clearing memory destroys that evidence before it can be read.

   Source: ASE A8 Engine Performance task list, General Engine Diagnosis (verify driver complaint and duplicate conditions); SAE J1979 services for pending DTCs and freeze frameReport a problem with this question

2. 2. A vacuum gauge on the intake manifold reads a steady 18 in Hg at idle. Held at a steady 2500 rpm, the reading climbs briefly and then falls off steadily to about 8 in Hg. What does this indicate?

   • A.A restricted exhaust, such as a plugged converter, that traps pressure at high rpm✓ Answer
   • B.A burned exhaust valve, which drops the needle sharply once each cylinder cycle
   • C.A leaking intake manifold gasket, which admits extra air and lowers vacuum at idle
   • D.Late ignition timing, which holds vacuum low and steady across the whole speed range

   A vacuum reading that is normal at idle but bleeds down as engine speed is held up is the classic signature of an exhaust restriction. The engine cannot push the increasing volume of exhaust past the blockage, backpressure builds, and less fresh charge can enter, so manifold vacuum falls. A gasket leak or retarded timing would show low vacuum at idle instead.

   Source: ASE A8 Engine Performance task list, General Engine Diagnosis (perform manifold vacuum tests and test for exhaust restriction)Report a problem with this question

3. 3. Cranking compression on a four-cylinder engine reads 190, 185, 60 and 188 psi against a specification of 190 psi. A few drops of oil added to cylinder 3 raise its reading to 175 psi. Technician A says the wet-test result points to worn rings or a worn cylinder wall. Technician B says the readings point to a head gasket leaking between cylinders 3 and 4. Who is right?

   • A.Technician B only
   • B.Technician A only✓ Answer
   • C.Neither Technician A nor Technician B
   • D.Both Technician A and Technician B

   Oil poured into the cylinder temporarily seals the rings, so a large rise on the wet test means the leak path was past the rings rather than past a valve or gasket. A head gasket failure between two cylinders lowers both of the adjacent cylinders, and here only cylinder 3 is low while cylinder 4 is normal.

   Source: ASE A8 Engine Performance task list, General Engine Diagnosis (perform cranking and running compression tests and interpret results)Report a problem with this question

4. 4. An engine cranks slowly. While the engine is cranking, the technician measures 0.6 volt between the battery negative post and the engine block. The service information allows a maximum of 0.2 volt across that path. What does the reading show?

   • A.The starter armature is shorted internally and is drawing far more current than normal
   • B.The battery is discharged and cannot deliver the cranking current the starter needs
   • C.The starter solenoid contacts are burned and are dropping voltage on the feed side
   • D.The ground path through the cable and its connections has excessive resistance✓ Answer

   A voltage drop test measures the voltage lost across a conductor while it carries its normal current, and anything above the published limit is unwanted resistance in that specific path. Because the meter was connected across the battery negative to block path only, the excess drop is in the ground cable or its terminals, not inside the starter or battery.

   Source: ASE A8 Engine Performance task list, General Engine Diagnosis (perform starter circuit voltage drop tests and interpret results)Report a problem with this question

5. 5. An engine sets a code for camshaft position timing over-advanced on bank 1 and idles roughly. The oil is well past its change interval and a heavier viscosity than the one specified was installed. What is the most likely cause?

   • A.The timing chain has jumped a tooth, so the camshaft is mechanically out of position
   • B.The crankshaft reluctor ring has slipped, shifting the timing reference signal
   • C.Oil sludge and heavy viscosity are restricting flow to the camshaft phaser control valve✓ Answer
   • D.The knock sensor is retarding ignition timing because of the fuel grade being used

   Variable valve timing phasers are hydraulic devices moved by engine oil that the control valve meters, so oil condition, level and viscosity are part of the system. Sludge or oil that is too thick restricts flow through the control valve screen, the phaser cannot be held at the commanded angle, and the controller sets a cam timing correlation code.

   Source: ASE A8 Engine Performance task list, General Engine Diagnosis (verify camshaft timing including variable valve timing; diagnose performance concerns caused by oil condition and viscosity)Report a problem with this question

6. 6. A cylinder 2 misfire code is stored on a coil-on-plug engine. The technician swaps the cylinder 2 coil with the cylinder 5 coil and clears the code. After a road test the cylinder 2 misfire code returns. What should the technician do next?

   • A.Replace the original coil anyway, because a weak coil misfires only in its own cylinder
   • B.Condemn the module's coil driver for cylinder 2, because the fault followed the wiring
   • C.Clear the code and release the vehicle, because one stored misfire needs no further work
   • D.Test cylinder 2's plug, injector and compression, since the misfire stayed put✓ Answer

   Swapping a component between cylinders is a substitution test: if the fault follows the part, the part is bad, and if the fault stays with the cylinder, the coil has been eliminated. The misfire remained on cylinder 2, so the remaining causes are that cylinder's spark plug, its injector or its mechanical condition.

   Source: ASE A8 Engine Performance task list, Ignition System Diagnosis (diagnose misfire and determine root cause of ignition-related concerns)Report a problem with this question

7. 7. On a secondary ignition waveform, cylinder 4 shows a firing line much higher than the other cylinders and a spark line that is shorter than the others. What does this pattern indicate?

   • A.A shorted coil primary winding, which limits how long the coil can be saturated
   • B.A rich mixture in that cylinder, which lowers the voltage needed to ionize the gap
   • C.High secondary resistance in that cylinder, from a wide gap or an open plug wire✓ Answer
   • D.A fouled plug in that cylinder, which shorts part of the secondary charge to ground

   The firing line height shows how much voltage the coil had to produce to jump the gap, and the spark line shows how long the spark burned with the energy left over. High resistance forces a tall firing line and consumes energy that would otherwise sustain the burn, so the spark line is short; a fouled plug or rich mixture produces the opposite pattern.

   Source: ASE A8 Engine Performance task list, Ignition System Diagnosis (inspect and test ignition secondary circuit using an oscilloscope)Report a problem with this question

8. 8. An engine cranks but does not start. There is no spark and no injector pulse, and the scan tool displays 0 rpm while the engine is cranking. What is the most likely cause?

   • A.A lost crankshaft position sensor signal, the main input for spark and injector timing✓ Answer
   • B.An open ignition coil primary winding, which kills spark on every cylinder at once
   • C.A plugged fuel filter that starves the rail during extended cranking periods
   • D.A failed camshaft position sensor, which by itself stops all spark and fuel delivery

   The crankshaft position sensor is the controller's primary engine speed and reference input, and the module will not command spark or injector pulses without it. A scan tool showing 0 rpm during cranking proves the controller is not receiving that signal, while a camshaft signal loss usually causes only extended cranking because most controllers can default to a non-sequential strategy.

   Source: ASE A8 Engine Performance task list, Ignition System Diagnosis (diagnose no-starting and hard starting concerns; test ignition system sensors)Report a problem with this question

9. 9. NOTE: This question asks for the exception. All of the following can produce spark knock under load EXCEPT:

   • A.Combustion chamber carbon deposits that raise compression and create hot spots
   • B.An exhaust gas recirculation valve that fails to flow, raising peak combustion temperature
   • C.An evaporative purge valve that stays closed and never feeds stored vapor to the intake✓ Answer
   • D.Ignition timing that is over-advanced for the octane rating of the fuel in the tank

   Spark knock is uncontrolled combustion caused by excessive heat or pressure in the chamber, so over-advanced timing, carbon deposits and a non-flowing recirculation valve all promote it. A purge valve that stays closed simply leaves stored vapor in the canister; it may set an evaporative system code or cause vapor odor, but it does not raise combustion temperature.

   Source: ASE A8 Engine Performance task list, Ignition System Diagnosis (diagnose spark knock and determine root cause) and Emissions Control Systems, exhaust gas recirculationReport a problem with this question

10. 10. An engine stores lean codes on both banks. At idle short-term fuel trim is +22% and long-term is +18%; at a steady 2500 rpm both fall to about +3%. What does this pattern indicate?

    • A.An unmetered air leak behind the airflow sensor, large next to idle airflow✓ Answer
    • B.A contaminated airflow sensor that under-reports intake air at every engine speed
    • C.Both upstream oxygen sensors biased lean by an exhaust leak ahead of the sensors
    • D.A weak fuel pump that cannot keep up with the volume demanded at higher engine load

    Fuel trim is positive when the controller is adding fuel to correct a lean condition. A fixed-size air leak is a large percentage of the small airflow at idle but a tiny percentage of the much larger airflow at 2500 rpm, so the correction is huge at idle and nearly disappears at speed. Fuel delivery restrictions and a dirty airflow sensor behave in the opposite way, getting worse as demand rises.

    Source: ASE A8 Engine Performance task list, Fuel, Air Induction and Exhaust Systems (analyze fuel trim and scan tool data; diagnose vacuum leaks and unmetered air)Report a problem with this question

11. 11. An engine sets a lean code on bank 1 only, and bank 2 fuel trims stay near zero at idle and at cruise. Technician A says a contaminated mass airflow sensor is the likely cause. Technician B says a leaking intake runner gasket on bank 1 could be the cause. Who is right?

    • A.Neither Technician A nor Technician B
    • B.Technician B only✓ Answer
    • C.Both Technician A and Technician B
    • D.Technician A only

    The airflow sensor measures all the air entering the engine, so a sensor that under-reports would drive both banks lean, not one. A fault confined to a single bank must have a bank-specific cause such as that bank's intake runner gasket, its injectors, or an exhaust leak upstream of that bank's oxygen sensor.

    Source: ASE A8 Engine Performance task list, Fuel, Air Induction and Exhaust Systems (analyze fuel trim data and diagnose incorrect air-fuel ratio concerns)Report a problem with this question

12. 12. Which statement correctly describes fuel pressure testing on an electronic returnless fuel system?

    • A.Rail pressure bleeds to zero at key off, so a leak-down test cannot be performed
    • B.Rail pressure is set by restriction in the return line, so a pinched return raises it
    • C.Rail pressure should climb roughly 7 to 10 psi when the regulator vacuum hose is pulled
    • D.Rail pressure does not respond to manifold vacuum, so volume must be tested as well✓ Answer

    An electronic returnless system regulates pressure at the tank, or by controlling pump speed, and has no manifold-vacuum reference line at the rail, so pressure does not rise when a vacuum hose is disconnected. Because a pump can hold static pressure and still fail to deliver enough fuel under demand, pressure and volume must both be measured.

    Source: ASE A8 Engine Performance task list, Fuel, Air Induction and Exhaust Systems (inspect and test fuel pressure regulation; perform fuel pressure and volume tests)Report a problem with this question

13. 13. An engine starts normally when cold. After a short hot soak it cranks a long time, then runs rough and rich for a few seconds. Rail pressure falls to zero within two minutes of shutdown. What is the most likely cause?

    • A.A purge solenoid stuck closed, which traps vapor in the canister during the soak
    • B.An intake air temperature sensor reading too warm, which leans the start mixture
    • C.A restricted fuel return line that traps high pressure in the rail after shutdown
    • D.A leaking injector or pump check valve that bleeds fuel into the intake during the soak✓ Answer

    Loss of residual rail pressure during the soak points to fuel escaping somewhere, and fuel that escapes through a leaking injector collects in the intake and floods the cylinder, which explains both the long crank and the brief rich running afterward. A pump check valve that leaks back to the tank causes the long crank without the flooding.

    Source: ASE A8 Engine Performance task list, Fuel, Air Induction and Exhaust Systems (diagnose hot or cold no-starting, hard starting and driveability concerns; inspect and test injectors)Report a problem with this question

14. 14. A vehicle stores a small evaporative leak code, defined as a leak equivalent to a 0.040 in orifice, with no driveability complaint. The fuel cap is in place and appears undamaged. What is the best next step?

    • A.Replace the fuel cap, because a cap is the only part that can set a small leak code
    • B.Top off the fuel tank and clear the code so that the leak monitor is forced to run
    • C.Introduce low-pressure smoke and look for smoke escaping at hoses and seals✓ Answer
    • D.Command the purge valve open with the scan tool and watch for a change in idle quality

    A small leak code only reports that the sealed system lost its test pressure or vacuum faster than the calibrated threshold; it does not identify which component leaks. Smoke testing pressurizes the system with a visible medium so the technician can find the actual leak point, which may be a hose, a seal, the canister, or the cap sealing surface.

    Source: ASE A8 Engine Performance task list, Emissions Control Systems, evaporative emissions controls (inspect, test and diagnose evaporative emissions control system)Report a problem with this question

15. 15. With the engine idling, a technician pinches the hose between the intake manifold and the positive crankcase ventilation valve. Idle speed and idle quality do not change at all. What does this result indicate?

    • A.The system is working normally, because a good system causes no change in idle
    • B.The passage between the manifold and the valve is plugged and no flow existed✓ Answer
    • C.The crankcase is over-pressurized and is forcing oil past the rear main seal
    • D.The valve is stuck open and is admitting extra air into the intake manifold

    A working crankcase ventilation system meters a measured amount of crankcase gas into the manifold, so blocking that flow must change idle speed or quality. When pinching the hose produces no change at all, the flow was already zero, which means the valve or the passage feeding it is plugged.

    Source: ASE A8 Engine Performance task list, Emissions Control Systems, positive crankcase ventilation (inspect, test and diagnose positive crankcase ventilation system)Report a problem with this question

16. 16. A catalyst efficiency code is stored for bank 1. On the scan tool the downstream oxygen sensor signal switches almost as fast and as widely as the upstream sensor. What should the technician do first?

    • A.Look for the fault that damaged the converter, such as a misfire or rich running✓ Answer
    • B.Clear the code and run the catalyst monitor twice before doing any other testing
    • C.Replace the downstream oxygen sensor, because a slow sensor usually sets this code
    • D.Replace the converter, because a mirroring downstream signal proves the substrate failed

    The monitor compares a rapidly switching upstream signal with a downstream signal that should stay nearly flat because a working converter stores and releases oxygen. A converter is almost always the victim of something else, so a misfire, a rich mixture, oil or coolant consumption, or an exhaust leak near the sensor must be found and corrected or the replacement will fail too.

    Source: ASE A8 Engine Performance task list, Emissions Control Systems, catalytic converter (inspect, test and diagnose catalytic converter performance and determine root cause)Report a problem with this question

17. 17. What information does freeze frame data contain?

    • A.The highest and lowest value each sensor reached during the current drive cycle
    • B.The operating conditions that were recorded when the fault first turned on the lamp✓ Answer
    • C.A rolling record of the last minutes of sensor data before the codes were cleared
    • D.The pass or fail status of every readiness monitor at the last drive cycle

    Freeze frame is a single snapshot of parameters such as engine speed, load, coolant temperature, fuel trim and vehicle speed, stored at the instant the fault that commanded the lamp was confirmed. Because it preserves the conditions present when the problem actually happened, it is the strongest evidence available for an intermittent complaint, and clearing codes erases it.

    Source: SAE J1979 E/E diagnostic test modes, freeze frame data service definitionReport a problem with this question

18. 18. Technician A says a scan tool's clear-codes command erases permanent diagnostic trouble codes along with the confirmed codes. Technician B says disconnecting the battery erases permanent codes and leaves all readiness monitors set to ready. Who is right?

    • A.Technician B only
    • B.Technician A only
    • C.Both Technician A and Technician B
    • D.Neither Technician A nor Technician B✓ Answer

    Permanent codes exist precisely so that a fault cannot be hidden by clearing memory: only the controller can erase one, and only after the associated monitor has run and passed on the required number of drive cycles. Removing battery power does not remove them either, and clearing codes resets readiness monitors to not ready, which causes an emissions test to be rejected.

    Source: SAE J1979 permanent DTC service definition; U.S. EPA on-board diagnostic requirements for permanent DTCs, 40 CFR Part 86Report a problem with this question

19. 19. Several codes set at the same time. Scan data shows manifold absolute pressure at 0.1 volt, throttle position at 0 percent and exhaust gas recirculation position at 0 volt, while coolant and intake air temperature read plausible values. What is the most likely cause?

    • A.A contaminated airflow sensor that pulls the shared signal ground low
    • B.A poor module ground connection, which drives every reading above normal
    • C.Three separate sensor failures that happen to share a mounting location
    • D.A short to ground on the 5 volt reference circuit that those three sensors share✓ Answer

    Pressure, throttle and valve position sensors are three-wire devices fed by a common regulated 5 volt reference from the controller. Losing that reference to a short or an open drives every sensor on the circuit to a rail value at once, so the pattern of several unrelated sensors reading zero together points to the shared circuit rather than to the sensors themselves.

    Source: ASE A8 Engine Performance task list, Computerized Engine Controls (diagnose sensor circuits; test and repair voltage supply and ground distribution circuits)Report a problem with this question

20. 20. A vehicle has a driveability complaint but no stored or pending trouble codes. Which on-board data is most useful for finding a component that is marginal but has not yet failed?

    • A.Mode $06 on-board test results, compared against the stored minimum and maximum✓ Answer
    • B.Mode $04 clear-codes data, which lists the monitors that failed before being erased
    • C.Mode $09 vehicle information, which reports the calibration identification numbers
    • D.Mode $03 stored trouble codes, read again after a short road test at part throttle

    On-board test results report the actual numeric value each monitor measured together with the limits it is judged against, so a component drifting toward its limit is visible before it fails the test and sets a code. The clear-codes service only erases data and returns no test results, and vehicle information returns identification data rather than measurements.

    Source: SAE J1979 E/E diagnostic test modes, on-board monitoring test results service definitionReport a problem with this question

21. 21. How does a wide-band air/fuel ratio sensor differ from a conventional zirconia oxygen sensor?

    • A.It measures exhaust gas temperature instead of oxygen to calculate the mixture
    • B.It generates a 0 to 1 volt signal that must cross 0.45 volt several times per second
    • C.It reports the actual ratio using a pumping current and shows a fixed voltage✓ Answer
    • D.It reports only rich or lean and is ignored by the module once the engine is warm

    A wide-band sensor uses a pump cell whose current is varied to hold the measuring cell at the stoichiometric point, and that current is proportional to how far the mixture is from stoichiometry. Many scan tools therefore display a nearly fixed voltage or a lambda value near 1.00 while the engine is in control, so judging one by switching activity leads to a wrong conclusion.

    Source: ASE A8 Engine Performance task list, Computerized Engine Controls (use scan tool and lab scope to test oxygen and air/fuel ratio sensors)Report a problem with this question

22. 22. The malfunction indicator lamp flashes while the vehicle is climbing a grade under load. Technician A says the flashing lamp indicates a misfire severe enough to damage the catalytic converter. Technician B says the driver should reduce load and stop driving the vehicle until it is repaired. Who is right?

    • A.Technician B only
    • B.Neither Technician A nor Technician B
    • C.Technician A only
    • D.Both Technician A and Technician B✓ Answer

    A flashing lamp is the standardized warning for a misfire rate high enough to overheat and destroy the converter, and unlike most emissions faults it is set on a single trip rather than after two. Raw fuel entering the converter burns in the substrate, so continuing to drive under load turns an ignition repair into a converter replacement.

    Source: U.S. EPA on-board diagnostic requirements for malfunction indicator lamp operation during catalyst-damaging misfire, 40 CFR Part 86; SAE J1979 misfire monitorReport a problem with this question

Practice questions written to the ASE Automobile & Light Truck (A-series) task lists and standard automotive service principles. ASE is a mark of the National Institute for Automotive Service Excellence; this site is not affiliated with or endorsed by ASE. Always follow the vehicle manufacturer's service information for specifications and procedures on a real repair. About the ASE tests →