22 A8 — Engine Performance Practice Questions & Answers

1. 1. A driver reports an intermittent hesitation during acceleration. Technician A says the first step is to verify the complaint, including a road test under the conditions the driver described. Technician B says that because no DTCs are stored, the complaint cannot be diagnosed and the PCM should be replaced. Who is right?

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

   Verification comes first because a fault you never duplicated cannot be confirmed as repaired, and the road-test conditions tell you which PIDs to watch. The absence of DTCs only means no monitor exceeded its failure threshold; many drivability faults stay inside PCM limits and set no code, so 'no codes' never justifies replacing the PCM.

   Source: ASE A8 Content Area A (General Diagnosis), Tasks A.1–A.3: verify driver's complaint, road test, and research service information before component replacementReport a problem with this question

2. 2. An engine idles smoothly with a steady 18 in.Hg on a vacuum gauge, but lacks power at highway speed. With engine speed held at 2,000 rpm, the gauge starts near 17 in.Hg and slowly falls to about 8 in.Hg. Which of these is the most likely cause?

   • A.An intake manifold vacuum leak
   • B.Retarded ignition timing
   • C.A restricted exhaust system✓ Answer
   • D.A weak fuel pump

   A gauge reading that starts normal and then decays while rpm is held is the classic restriction signature: exhaust backpressure builds behind the restriction faster than the engine can pump it out, so the cylinders cannot scavenge and manifold vacuum falls. A vacuum leak or retarded timing produces a low but steady reading at idle rather than a progressive drop at held rpm.

   Source: ASE A8 Content Area A: vacuum gauge interpretation — steady needle drop at held rpm indicates exhaust restriction; ASE A8 Content Area C exhaust restriction testingReport a problem with this question

3. 3. A vehicle cranks slowly and the driver reports occasional no-starts followed by rough running. The battery is fully charged and tests good. With a DMM connected between the battery negative post and the engine block during cranking, the meter reads 0.6 V. What does this reading indicate?

   • A.Normal voltage drop; no fault is indicated
   • B.A shorted armature inside the starter
   • C.A discharged battery
   • D.Excessive resistance in the negative (ground) side of the starting circuit✓ Answer

   A voltage-drop test measures the voltage consumed by resistance in a conductor while current actually flows, so any reading across a cable is voltage the load never receives. A healthy ground path drops only a couple of tenths of a volt during cranking; 0.6 V means corroded or loose connections are stealing energy from the starter, and the same poor ground can skew PCM sensor references and cause driveability complaints.

   Source: ASE A8 Content Area A, electrical tasks: perform starting and charging circuit voltage drop tests; battery and connection conditionReport a problem with this question

4. 4. The MIL is on with P0128 'Coolant Temperature Below Thermostat Regulating Temperature'. The driver reports poor fuel economy and weak heater output. Which of these best explains the code and the symptoms?

   • A.A restricted catalytic converter raising exhaust backpressure
   • B.A thermostat stuck open, so ECT never reaches the regulating temperature and the PCM extends warm-up enrichment✓ Answer
   • C.A purge valve stuck closed, trapping vapor in the canister
   • D.A thermostat stuck closed, causing the engine to overheat

   The PCM sets P0128 when ECT fails to climb to the expected regulating temperature within a calculated time and airflow, which happens when coolant circulates continuously through the radiator. Because fuel strategy is temperature-based, a cold engine stays in warm-up enrichment longer and delivers a richer mixture, producing poor economy and a cool heater.

   Source: ASE A8 Content Area A: cooling system effect on engine performance; SAE J2012 generic DTC P0128 definitionReport a problem with this question

5. 5. An engine cranks but will not start. The scan tool shows no engine rpm while cranking, there is no spark at any cylinder, and no injector pulse can be detected. Fuel pressure is within specification. Which of these is the most likely cause?

   • A.One ignition coil with an open primary winding
   • B.A failed CKP sensor or its circuit✓ Answer
   • C.A failed fuel pump module
   • D.An open MAF sensor circuit

   The PCM times both spark and injector pulse from the crankshaft position (CKP) signal, so losing that one input suppresses both outputs at once and also leaves the rpm PID at zero. A MAF fault would allow a default airflow calculation and still produce spark, a fuel pump problem would not stop spark, and a single open coil would cause a misfire rather than a complete no-start.

   Source: ASE A8 Content Area B: diagnose no-start related to the ignition system; CKP is the primary reference for spark and injector synchronizationReport a problem with this question

6. 6. An EI (coil-on-plug) engine has P0303 'Cylinder 3 Misfire Detected'. All coils are supplied by one fused feed. Swapping the coil from cylinder 3 with the coil from cylinder 4 does not move the misfire. With the engine running, a voltage drop of 3.5 V is measured across the feed wire to coil 3. Which statement is correct?

   • A.An open CKP circuit would explain a single-cylinder misfire
   • B.High resistance in the feed circuit to coil 3 is starving that coil and causing the misfire✓ Answer
   • C.An open fuse in the shared feed would cause a misfire on cylinder 3 only
   • D.The PCM must be replaced because the coil swap did not move the misfire

   A coil must receive full system voltage to saturate its primary winding and store enough energy for a strong spark, so resistance in that single feed leg lowers available voltage and weakens only that cylinder's spark. The swap test already ruled the coil itself out, an open shared fuse would kill every coil it supplies, and an open CKP causes a complete no-start rather than one misfiring cylinder.

   Source: ASE A8 Content Area B: diagnose ignition primary circuit faults using wiring schematics and voltage drop testingReport a problem with this question

7. 7. While climbing a grade under load, a vehicle's MIL begins to flash and P0302 'Cylinder 2 Misfire Detected' is stored. What does the flashing MIL tell the technician?

   • A.The PCM has entered a default fuel strategy and the misfire can be ignored
   • B.The code is only pending and will not illuminate the MIL until a second trip
   • C.A monitor has not yet completed and readiness is incomplete
   • D.The misfire rate is high enough to damage the catalytic converter, so load should be reduced and the fault repaired before further driving✓ Answer

   A misfire severe enough to pass unburned fuel into the exhaust lets that fuel burn inside the converter, and the resulting overtemperature destroys the substrate. That is why a catalyst-damaging (Type A) misfire flashes the MIL and sets the DTC on a single trip, instead of the two consecutive trips required for an emissions-threshold (Type B) fault.

   Source: ASE A8 Content Areas B and E: misfire monitor — Type A (one-trip, flashing MIL, catalyst damaging) vs Type B (two consecutive trips)Report a problem with this question

8. 8. A DSO is connected to secondary ignition. On one cylinder the firing line is much higher than the others and the spark line is short and steeply sloped. Which of these is the most likely cause?

   • A.A fouled spark plug shorting the secondary to ground
   • B.Low primary supply voltage to all of the coils
   • C.Excessive resistance in that cylinder's secondary circuit, such as a worn spark plug with an excessive gap✓ Answer
   • D.An excessively rich mixture in that cylinder only

   Firing-line height equals the voltage required to ionize the gap, so a wide gap, an open plug wire, or a corroded connection raises that demand. Because the coil's stored energy is then consumed at a higher voltage, less is left to sustain the arc, which is why the spark line is short and steep; a fouled plug produces the opposite pattern — a low firing line with a long spark line.

   Source: ASE A8 Content Area B: interpret ignition secondary waveforms — firing line height indicates required firing voltage, spark line length indicates available energyReport a problem with this question

9. 9. A MAF-equipped SFI engine has P0171 'System Too Lean (Bank 1)'. At idle STFT is +22% and LTFT is +25%; at a steady 2,500 rpm cruise both trims fall to about +3%. Which of these is the most likely cause?

   • A.A leaking injector adding extra fuel
   • B.A contaminated MAF under-reporting airflow at all speeds
   • C.A restricted fuel filter limiting fuel volume
   • D.Unmetered air entering downstream of the MAF, such as a vacuum leak✓ Answer

   Positive fuel trim means the PCM is lengthening injector pulse width to correct a lean condition, and a fixed-size air leak represents a large percentage of the small airflow present at idle. As the throttle opens, metered airflow grows and the same leak becomes a small fraction of the total, so the trims return toward zero — a fuel-supply restriction or a skewed MAF would stay lean at cruise as well.

   Source: ASE A8 Content Area C: interpret short and long term fuel trim — lean trim only at idle indicates unmetered airReport a problem with this question

10. 10. A MAF-equipped SFI engine has P0171 'System Too Lean (Bank 1)'. LTFT is +18% at idle and +20% at cruise, and the engine stumbles under heavy acceleration. Fuel pressure is within specification at idle but falls well below specification during a loaded acceleration test. Which of these is the most likely cause?

    • A.A vacuum leak at the PCV hose
    • B.A fuel delivery restriction, such as a plugged filter or weak pump, limiting volume under demand✓ Answer
    • C.A purge valve stuck open
    • D.An exhaust leak downstream of the catalytic converter

    Lean trim that persists across idle, cruise and load points to a fuel-side shortfall rather than unmetered air, because an air leak's effect shrinks as airflow rises. Pressure can look correct at low demand and still be inadequate when flow increases, which is why volume must be tested along with pressure; a restriction shows up as pressure collapsing under load.

    Source: ASE A8 Content Area C: fuel system pressure and volume testing; lean trim at all load ranges indicates fuel delivery shortfallReport a problem with this question

11. 11. A returnless EFI system, with the pressure regulator located in the tank-mounted fuel pump module, is being diagnosed for a rich running complaint with black smoke. Fuel rail pressure measures higher than specification at idle. Which of these could be the cause?

    • A.A leaking fuel pump check valve
    • B.A restricted fuel filter
    • C.A restricted (partially plugged) injector
    • D.A failed fuel pressure regulator✓ Answer

    In a returnless system the regulator is the only device that sets maximum rail pressure by bleeding excess fuel back inside the tank module, so pressure above specification can only come from a regulator that is not relieving. Restrictions and leaks can only reduce or bleed off pressure, and excessive rail pressure over-fuels every injection event, driving the mixture rich.

    Source: ASE A8 Content Area C: fuel pressure regulator diagnosis on returnless EFI systemsReport a problem with this question

12. 12. A MAF-equipped engine hesitates on acceleration. LTFT is about +18% at cruise, no vacuum leaks are found with a smoke test, fuel pressure and volume test good, and the MAF PID reads noticeably lower grams per second than expected at wide open throttle. Which of these is the most likely cause?

    • A.Excessive fuel pressure
    • B.A purge valve stuck open
    • C.A contaminated MAF sensor that under-reports actual airflow✓ Answer
    • D.A leaking injector

    On a mass airflow system the PCM calculates base injector pulse width directly from measured airflow, so a hot-wire element coated with dirt or oil loses heat more slowly and reports less air than is actually entering. The base fuel command is then too small, the oxygen or A/F ratio sensor reports lean, and the PCM must add large positive trim to compensate — the other choices would drive the mixture rich and produce negative trim.

    Source: ASE A8 Content Areas C and E: MAF sensor diagnosis; under-reported airflow produces positive fuel trimReport a problem with this question

13. 13. A vehicle has P0442 'Evaporative Emission System Leak Detected (Small Leak)' stored, with no drivability complaint. Which of these is the most likely cause?

    • A.A torn or hardened fuel cap seal✓ Answer
    • B.An open fuel tank pressure (FTP) sensor circuit
    • C.A restricted canister vent valve
    • D.A purge solenoid stuck closed

    A leak DTC is set only when the sealed EVAP system cannot hold the pressure or vacuum the monitor applies, so it requires an actual breach in the sealed volume — and the cap seal is the most common small breach. A stuck-closed purge or a restricted vent blocks flow and sets purge or vent flow codes, while an open FTP circuit sets a sensor circuit code, because none of those three opens the system to atmosphere.

    Source: ASE A8 Content Area D (EVAP): distinguish leak DTCs from flow and circuit DTCs; SAE J2012 P0442 definitionReport a problem with this question

14. 14. A vehicle idles roughly and occasionally stalls immediately after a hot restart. Scan data shows LTFT of about -15% at idle, returning to near 0% at cruise. No vacuum leaks are found and fuel pressure is correct. Which of these is the most likely cause?

    • A.An EVAP purge valve stuck open, allowing unmetered fuel vapor into the intake manifold at idle✓ Answer
    • B.A plugged PCV orifice
    • C.A leaking fuel cap seal
    • D.A restricted canister vent valve

    Stored vapor is unmetered fuel, and a hot soak loads the canister heavily, so a purge valve that leaks vapor at idle enriches the small idle charge enough to cause a rough idle or stall. Negative long term fuel trim confirms the PCM is subtracting fuel to compensate for a rich condition, and the trim returns to normal at cruise because the same vapor flow is a small fraction of the much larger airflow.

    Source: ASE A8 Content Area D (EVAP): purge solenoid diagnosis; rich condition produces negative fuel trimReport a problem with this question

15. 15. This question contains the word EXCEPT. Read the question carefully before choosing your answer. A vehicle has P0401 'Exhaust Gas Recirculation Flow Insufficient Detected'. All of these could cause this DTC EXCEPT:

    • A.An EGR valve stuck open✓ Answer
    • B.An exhaust leak near the EGR pickup point
    • C.An electrically open EGR control solenoid
    • D.A carbon-restricted EGR passage in the intake manifold

    P0401 is set when the PCM commands recirculation and the feedback it monitors shows less flow than expected, which a plugged passage, an open control solenoid, or an exhaust leak bleeding off pressure can all produce. A valve stuck open does the opposite — it flows exhaust when none is commanded, causing rough idle, hesitation or stalling and an excessive-flow DTC.

    Source: ASE A8 Content Area D (EGR): diagnose insufficient vs excessive EGR flow; SAE J2012 P0401 definitionReport a problem with this question

16. 16. With the engine idling, a technician removes the PCV valve from the valve cover and plugs the opening in the cover. Idle speed and quality do not change at all. What does this result indicate?

    • A.The crankcase is being over-pressurized by excessive blowby
    • B.The PCV system is operating normally
    • C.The PCV valve is stuck open and flowing too much
    • D.Flow through the PCV circuit is already blocked, such as a plugged hose between the intake manifold and the valve✓ Answer

    The PCV circuit meters a calibrated amount of crankcase gas into manifold vacuum, so sealing a functioning circuit removes that flow and must produce a noticeable idle change. No change at all means no flow was occurring in the first place — a stuck-open valve would have been flowing more than normal and would have produced an obvious change when plugged.

    Source: ASE A8 Content Area D (PCV): perform PCV system functional/flow testReport a problem with this question

17. 17. A technician retrieves freeze frame data while diagnosing an intermittent emissions fault. What does freeze frame data provide?

    • A.The operating conditions recorded at the instant the emissions-related DTC set✓ Answer
    • B.The measured test values and pass/fail limits for each on-board monitor
    • C.A continuously updating live stream of current sensor values
    • D.A list of DTCs that cannot be erased with a scan tool

    Freeze frame is a single stored snapshot of parameters such as engine rpm, calculated load, ECT, vehicle speed and fuel trim, captured at the moment the DTC matured. It is valuable because it tells the technician the exact conditions to recreate on a road test, which is how an intermittent fault is duplicated instead of guessed at.

    Source: ASE A8 Content Area E: retrieve and interpret freeze frame data; SAE J1979 Service $02Report a problem with this question

18. 18. A repair has been completed and a permanent DTC (SAE J1979 Mode $0A) is still present. Technician A says clearing codes with a scan tool will remove the permanent DTC. Technician B says disconnecting the battery will remove it. Who is right?

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

    Permanent DTCs exist specifically so a fault cannot be hidden by erasing memory, so neither a scan tool clear nor a power interruption removes them. They self-clear only after the monitor associated with that DTC actually runs its enable criteria and passes, which is why the vehicle must be driven until the relevant monitor completes before the repair can be considered verified.

    Source: ASE A8 Content Area E: OBD II permanent DTCs; SAE J1979 Service $0AReport a problem with this question

19. 19. On a running engine, scan data shows the MAP sensor at 5.0 V (implausibly high), the TP sensor near 4.9 V with the throttle closed, and ECT reading -40 degrees. Which of these is the most likely cause?

    • A.An open in the 5-volt reference circuit shared by those sensors
    • B.An open in the sensor ground circuit shared by those sensors✓ Answer
    • C.An open CKP sensor circuit
    • D.Three separate failed sensors

    These sensors are voltage dividers fed by the PCM's 5-volt reference, so if the shared ground opens no current can flow through the divider and every affected signal line is pulled up toward reference voltage — high MAP, high TP and a full-cold ECT. Losing the 5-volt reference instead makes those same PIDs read 0 V, and one shared-circuit fault is always more probable than three sensors failing at once.

    Source: ASE A8 Content Area E: diagnose sensor reference and ground distribution circuits using scan tool and DMMReport a problem with this question

20. 20. A technician wants to evaluate a system that is marginal but has not yet set a DTC. Which OBD II service provides the actual on-board monitoring test values along with their minimum and maximum limits?

    • A.Mode $09, vehicle information such as VIN and CALID
    • B.Mode $08, bidirectional component control
    • C.Mode $06, on-board monitoring test results✓ Answer
    • D.Mode $02, freeze frame data

    Mode $06 reports each monitor's test identifier with the measured result and the pass/fail limits the PCM applied, so a technician can see how close a system came to failing. That makes it the tool for catching a degrading catalyst, EVAP system or sensor before the threshold is crossed and a DTC is stored, and for confirming a repair moved the value well inside limits.

    Source: ASE A8 Content Area E; SAE J1979 Service $06 (on-board monitoring test results)Report a problem with this question

21. 21. A vehicle has P0420 'Catalyst System Efficiency Below Threshold (Bank 1)'. Scan data shows the downstream oxygen sensor switching almost identically to the upstream sensor. Technician A says the converter should be replaced now. Technician B says exhaust leaks, misfire and a lazy or contaminated downstream sensor should be ruled out first. Who is right?

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

    The catalyst monitor never measures the converter directly; it infers oxygen storage capacity by comparing downstream sensor activity with upstream activity, so anything that adds oxygen or fuel to the exhaust stream can mimic a dead converter. An exhaust leak ahead of a sensor draws in fresh air, a misfire dumps raw fuel and oxygen past the substrate, and a contaminated sensor reports false activity, so those must be ruled out before condemning the converter — and a converter that truly failed usually failed because of one of them.

    Source: ASE A8 Content Areas D and E: catalyst monitor operation (non-continuous, upstream vs downstream sensor comparison); root cause diagnosis before converter replacementReport a problem with this question

22. 22. Compared with a conventional zirconia heated oxygen sensor (HO2S), how does an upstream air/fuel (A/F) ratio sensor behave?

    • A.It is used only downstream, to evaluate catalytic converter efficiency
    • B.It reports the actual air/fuel ratio over a wide range instead of simply switching rich or lean around 14.7:1✓ Answer
    • C.It switches faster between 0.1 V and 0.9 V but still only indicates rich or lean
    • D.It operates only in open loop, before the engine reaches operating temperature

    A wideband A/F ratio sensor uses a pump cell whose current is controlled to hold the sensing cell at the stoichiometric point, and the PCM reads that pump current as a nearly linear measure of how rich or lean the mixture actually is. That extra information lets the PCM correct precisely and quickly, whereas a conventional zirconia sensor only crosses a switch point at 14.7:1 and is therefore best suited downstream, where its switching activity is compared with the upstream signal for the catalyst monitor.

    Source: ASE A8 Content Areas C and E: oxygen sensor and A/F ratio sensor operation; upstream fuel control vs downstream catalyst monitoringReport 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 →