22 Fuel Injection & Electronic Controls Practice Questions & Answers

1. 1. A high-pressure common rail engine cranks but will not start. A manual gauge teed into the rail shows pressure far below the value the service data lists for cranking. Which fault best explains this?

   • A.A rail pressure control valve stuck open, draining fuel to the tank✓ Answer
   • B.An open injector solenoid control circuit on one cylinder of the engine
   • C.An open rail pressure sensor signal circuit reporting a false low reading
   • D.A restricted injector nozzle tip on one cylinder limiting spray volume

   The manual gauge proves the pressure is genuinely low, so a sensor or reporting fault cannot be the cause — an open sensor circuit falsifies the displayed value without changing actual rail pressure. Only something that controls or bleeds rail pressure, such as a pressure control valve stuck open, drops the whole rail; a single restricted nozzle or one open injector circuit affects one cylinder and cannot collapse rail pressure.

   Source: ASE A9 Test Specification, Area F (Fuel System Diagnosis and Repair) — high-pressure common rail pressure control taskReport a problem with this question

2. 2. On a common rail system, what is the main purpose of the pilot (pre) injection event ahead of the main event?

   • A.It bleeds trapped air out of the rail before the main event is delivered
   • B.It delivers the bulk of the fuel that produces the torque the driver asks for
   • C.It shortens ignition delay so the pressure rise is gentler and quieter✓ Answer
   • D.It raises exhaust gas temperature so a regeneration can burn stored soot

   A small pilot charge starts combustion before the main quantity arrives, so less fuel is pre-mixed during the ignition delay period. That lowers the rate of cylinder pressure rise, which is what produces diesel knock, and it also lowers peak combustion temperature and NOx formation.

   Source: ASE A9 Test Specification, Area F — multiple injection events on electronically controlled common rail systemsReport a problem with this question

3. 3. On a common rail system, a post injection event late in the cycle is commanded mainly to:

   • A.scavenge residual exhaust gas from the cylinder before the intake opens
   • B.raise exhaust gas temperature so the oxidation catalyst can support a regeneration✓ Answer
   • C.shorten ignition delay so the main event burns quietly at idle
   • D.cool the piston crown by wetting it with fuel after peak cylinder pressure

   Fuel injected late in the cycle does not contribute useful torque; it passes out as unburned hydrocarbon and is oxidized in the diesel oxidation catalyst, and that exothermic reaction raises the temperature entering the particulate filter high enough to burn accumulated soot. Cooling and ignition-delay effects belong to other injection events.

   Source: ASE A9 Test Specification, Area F — multiple injection events; Area E aftertreatment regeneration strategyReport a problem with this question

4. 4. Technician A says that on a cam-driven electronic unit injector system, available injection pressure rises with engine speed. Technician B says that on a high-pressure common rail system, injection pressure is controlled by the module independently of engine speed. Who is right?

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

   A unit injector is pressurized by a cam-driven plunger, so plunger velocity and the pressure it can generate climb with engine speed. A common rail stores pressure in the rail ahead of the injectors, and the module holds a commanded pressure through the metering and pressure control valves, so full injection pressure is available even at cranking or low idle.

   Source: ASE A9 Test Specification, Area F — separate tasks for electronic unit injector and high-pressure common rail systemsReport a problem with this question

5. 5. A hydraulic electronic unit injector engine cranks normally but will not start. Scan data shows a high commanded injection pressure regulator duty cycle while actual injection control pressure stays far below the desired value. Which is the most likely cause?

   • A.A leak in the high-pressure oil circuit, such as a standpipe✓ Answer
   • B.An injector calibration code that was never entered after the repair
   • C.A glow plug circuit that is open on more than one cylinder
   • D.A low supply pressure reaching the inlet of the injection system

   In this architecture engine oil is the actuating medium: the high-pressure oil pump supplies it, the regulator controls it, and the control pressure sensor reports it. A high regulator command with low actual pressure means the circuit cannot hold what the pump is producing, so a leak in the high-pressure oil circuit is indicated; without minimum control pressure the injectors cannot fire at all.

   Source: ASE A9 Test Specification, Area F — hydraulic electronic unit injector (HEUI) system diagnosisReport a problem with this question

6. 6. In a hydraulic electronic unit injector system, the high-pressure engine oil supply is used to:

   • A.lubricate the injector plunger so the cam-driven pumping element does not scuff
   • B.hold the nozzle closed until the solenoid releases it at the timed moment
   • C.cool the injector tip so the nozzle holes stay free of carbon at high load
   • D.act on an intensifier piston inside the injector to pressurize the fuel for injection✓ Answer

   This system has no cam-driven pumping element and no rail; pressurized engine oil acts on the large face of an intensifier piston, and the small face multiplies that pressure into the fuel below it. That is why oil condition, level and viscosity directly control whether the engine will start and how it runs.

   Source: ASE A9 Test Specification, Area F — HEUI system operation and inspectionReport a problem with this question

7. 7. On an electronic pump-line-nozzle system, injection at the cylinder begins when:

   • A.the ECM energizes a solenoid built into the nozzle holder at that cylinder
   • B.the rail pressure sensor tells the ECM that the target rail pressure is reached
   • C.the line pressure lifts the nozzle valve off its seat against spring force✓ Answer
   • D.the intensifier piston in the injector body strokes down onto the fuel

   Pressure is created at the injection pump and travels through the high-pressure line to a hydraulically opened nozzle; when line pressure exceeds the nozzle opening pressure set by the spring, the valve lifts and spray begins. Electronic control in this architecture meters at the pump, so pump-to-engine timing determines when injection occurs.

   Source: ASE A9 Test Specification, Area F — electronic pump-line-nozzle (PLN-E) system diagnosisReport a problem with this question

8. 8. An electronic unit injector engine runs rough and smokes, and the complaint appeared right after the front timing cover was serviced. Which check should be made first?

   • A.Verify the timing of the gear train driving the camshaft and injectors✓ Answer
   • B.Enter fresh calibration codes for all of the injectors in the engine
   • C.Measure the voltage drop across the injector harness ground at idle
   • D.Perform an injector return volume test on every cylinder at rated speed

   Unit injectors are pressurized by the engine camshaft, so injection timing is mechanical timing; a drive gear indexed one tooth off after cover service changes when every injector pressurizes and produces rough running and smoke. Diagnosis starts with what the previous repair disturbed rather than with component testing.

   Source: ASE A9 Test Specification, Area F — electronic unit injector (EUI) system; Area B drive gear train timingReport a problem with this question

9. 9. A common rail engine is hard to start and rail pressure falls short of the commanded value under load. Return (leak-off) volume is measured at each injector at the specified speed and pressure, and one injector returns roughly four times the volume of the others. This indicates that the injector:

   • A.has an open solenoid winding and is not being energized at all
   • B.has a restricted nozzle tip that blocks the fuel it should spray
   • C.was installed with the wrong calibration code written into the module
   • D.is leaking internally and is bleeding rail pressure back to the return circuit✓ Answer

   Return volume is fuel that entered the injector but did not leave through the nozzle, so an injector returning far more than its neighbors is passing fuel internally past worn control valve or nozzle clearances. That internal leakage is also a load on the pump, which is why rail pressure falls short and starting is hard; the comparison is valid only against the volume the service data specifies.

   Source: ASE A9 Test Specification, Area F — injector return (leak-off) testing on high-pressure common rail systemsReport a problem with this question

10. 10. A scan tool cylinder contribution test on a common rail engine shows one cylinder contributing far less than the rest, and the fuel correction value for that cylinder is at its limit. Before condemning the injector, the technician should:

    • A.replace the injector and enter its code, then repeat the contribution test
    • B.test that cylinder's compression, since a mechanical fault looks the same✓ Answer
    • C.clear the stored codes and road test the vehicle to see whether the fault returns
    • D.measure the resistance of that injector's solenoid winding at the connector

    A contribution or balance test only reports that one cylinder is producing less than the others; it does not say why. Low compression from a burned valve, worn rings or a head gasket leak produces the same low contribution and the same extreme correction value as a bad injector, so the mechanical condition must be ruled out before a fuel component is replaced.

    Source: ASE A9 Test Specification, Area F — cylinder contribution/balance testing and injector diagnosisReport a problem with this question

11. 11. New injectors are installed on a common rail engine and it now runs rough, smokes and has driveability complaints. One step was left out of the job. Which step was most likely skipped?

    • A.Performing a forced regeneration of the particulate filter after the repair
    • B.Measuring the voltage drop on the injector power feed after the installation
    • C.Writing each injector's calibration code into the module after installation✓ Answer
    • D.Relearning the accelerator pedal position sensor idle voltage after the repair

    Each injector is flow-tested at the factory and its individual trim data is printed on the body as a calibration code. Until that code is written to the module and matched to the cylinder position, the module keeps applying the old injector's compensation values, so delivered quantity is wrong at every cylinder and the engine smokes and runs rough.

    Source: ASE A9 Test Specification, Area F — injector calibration code entry and control module relearnReport a problem with this question

12. 12. Technician A says injector calibration codes must be entered when an injector is moved from one cylinder to another. Technician B says the codes must also be entered when the engine control module itself is replaced. Who is right?

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

    The code is stored against a cylinder position, not against the injector itself, so relocating an injector leaves the module compensating the wrong hardware. A replacement module arrives with no injector data at all, so every code must be written before the engine will fuel correctly.

    Source: ASE A9 Test Specification, Area F — injector calibration codes and control module relearn proceduresReport a problem with this question

13. 13. A common rail engine sets an intermittent injector circuit code that follows one connector. The connector is unlatched and the terminals look clean. What is the best next step?

    • A.Replace the injector on that circuit and clear the stored code
    • B.Perform a terminal drag test and inspect the seals and locks for damage✓ Answer
    • C.Apply dielectric grease to the terminals and reconnect the harness
    • D.Measure the resistance of the harness with the connector unlatched

    A clean-looking terminal can still have lost spring tension, and a stretched terminal makes contact at rest but opens under vibration and thermal movement, which is exactly what an intermittent code describes. A drag test checks that tension directly, and damaged seals or an unseated lock let water in and allow the terminal to back out.

    Source: ASE A9 Test Specification, Area F — connector, terminal, harness, seal and lock inspection and serviceReport a problem with this question

14. 14. After reflashing an engine control module, which scan tool information confirms that the new calibration actually loaded?

    • A.The freeze frame data stored with the most recently set diagnostic trouble code
    • B.The on-board monitor test results for the affected emission monitors
    • C.The calibration identification and verification numbers in the module✓ Answer
    • D.The absence of any pending diagnostic trouble codes after a drive cycle

    The generic vehicle information service reports the calibration identification and the calibration verification number, which are the module's own record of which software it is running and a checksum of that software. Comparing those values to the ones the service information lists for the update is the only direct proof the reflash took; the absence of codes proves nothing about software level.

    Source: SAE J1979 Service $09 (vehicle information: CALID/CVN); ASE A9 Area F — verify and update software calibrationReport a problem with this question

15. 15. A diesel engine's fuel system fault was repaired and the codes were cleared, but one code remains and will not clear with the scan tool or by disconnecting the battery. This code:

    • A.is a manufacturer-specific code that needs a factory tool to be cleared
    • B.is a pending code that clears after two consecutive failed drive cycles
    • C.is a permanent code that clears only after the monitor runs and passes✓ Answer
    • D.is proof the module has failed and must be replaced and recalibrated

    Permanent codes exist so a repair cannot be hidden by clearing memory; they are stored separately and are immune to the clear command and to power loss. The module erases one only after the associated monitor has actually run and passed, which is why the correct response is to complete the drive cycle rather than to keep trying to clear it.

    Source: SAE J1979 Service $0A (permanent DTCs); ASE A9 Area A — scan tool DTC workflowReport a problem with this question

16. 16. An intermittent rail pressure deviation code is stored but the fault is not present now. Which scan tool information best shows the conditions that existed when the code set?

    • A.The freeze frame data the module captured at the moment the code set✓ Answer
    • B.The current live data list read while the engine idles in the bay
    • C.The list of pending codes that have not matured into stored codes
    • D.The on-board monitor test results for the last completed cycle

    Freeze frame is a snapshot of engine load, speed, temperature, fuel data and related parameters recorded at the instant the code matured, so it tells the technician the operating condition to reproduce during a road test. Live data taken at idle in the bay describes a condition in which the fault is not occurring and cannot substitute for it.

    Source: SAE J1979 Service $02 (freeze frame data); ASE A9 Area A — record freeze frame and operational dataReport a problem with this question

17. 17. Technician A says a good resistance reading on an injector feed wire proves the circuit can carry the injector's current. Technician B says a voltage drop test must be made with the circuit switched off so the meter is not damaged. Who is right?

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

    An ohmmeter pushes only a few milliamps, so a corroded splice or a partly broken strand can read near zero ohms and still starve the circuit once real current flows. A voltage drop test is valid only with the circuit energized and carrying its normal load, because the drop being measured exists only while current flows.

    Source: ASE A9 Test Specification, Area F — measure voltage, voltage drop, amperage and resistance with a DMMReport a problem with this question

18. 18. An injector solenoid circuit is suspected of having high resistance. Which measurement most directly shows that the circuit is not delivering full current to the solenoid?

    • A.Voltage measured at the module connector with the key on
    • B.Current draw read with an inductive clamp on the circuit✓ Answer
    • C.Resistance measured across the solenoid with the connector removed
    • D.Continuity checked through the harness with the battery off

    Current is the quantity that actually operates the solenoid, and unwanted resistance anywhere in the feed or ground limits it, so measuring current while the circuit works is the direct evidence. Static resistance and continuity checks are made with no load and can look normal on a circuit that collapses under operating current.

    Source: ASE A9 Test Specification, Area F — DMM current measurement in electronic control circuitsReport a problem with this question

19. 19. All of the following statements about a stored diagnostic trouble code are true EXCEPT:

    • A.The code must be verified as fixed by clearing it and re-testing
    • B.The code identifies a circuit or a condition the module has detected
    • C.The code should be recorded with freeze frame data before it is cleared
    • D.The code names the specific failed part that must be replaced to fix the fault✓ Answer

    A module can only report what its own monitoring strategy sees on a circuit or from a modeled condition, so a code points at a circuit, not at a part. An injector circuit code, for example, can be caused by the injector, by the harness, by a terminal, by a ground or by the driver inside the module, and pinpoint testing is what separates them.

    Source: ASE A9 Test Specification, Areas A and F — DTC interpretation and pinpoint testing before component replacementReport a problem with this question

20. 20. Before loosening any fitting on the high-pressure side of a common rail system, the technician must:

    • A.loosen the fitting slowly with the engine running to purge trapped air
    • B.shut the engine off and relieve rail pressure as the procedure states✓ Answer
    • C.run the engine at idle so that fuel can drain out of the loosened fitting
    • D.cover the fitting with a shop rag and open it while the rail is charged

    Common rail pressure is high enough that escaping fuel forms a cutting jet that injects through skin and causes a fluid injection injury, which is a surgical emergency even when the entry wound looks trivial. A rag does not stop the jet, so the rail must be depressurized by the published procedure and given the specified wait time before any fitting is opened.

    Source: ASE A9 Test Specification, Area F — high-pressure line, fitting and transfer tube service precautionsReport a problem with this question

21. 21. A common rail engine has a dead miss on one cylinder. Cutting out that injector's command with the scan tool produces no change in idle quality. This most nearly indicates that:

    • A.the cylinder was already producing no power before the test✓ Answer
    • B.the calibration code for that injector was entered incorrectly
    • C.the rail pressure control valve cannot hold the commanded pressure
    • D.the injector on that cylinder is leaking fuel internally

    Disabling a healthy cylinder always degrades idle, because its contribution is removed. When disabling produces no change, that cylinder was contributing nothing already, and the test has confirmed the misfire location without saying whether the cause is fuel, compression or the electrical circuit — that still has to be separated by further testing.

    Source: ASE A9 Test Specification, Area F — scan tool bidirectional injector cut-out and cylinder contribution testingReport a problem with this question

22. 22. A rail pressure sensor's 5-volt reference circuit is open. With the key on, the scan tool will most likely show:

    • A.a rail pressure reading at or near zero and a stored sensor circuit code✓ Answer
    • B.a rail pressure reading that is normal with no stored codes at all
    • C.a rail pressure reading pinned at the highest value in the range
    • D.a rail pressure reading that drifts slowly with engine temperature

    A three-wire pressure sensor builds its signal from the reference voltage, so with the reference open there is nothing to divide and the signal falls toward ground, which the module scales as minimum pressure and flags as an out-of-range-low circuit fault. Knowing how an open, a short to ground and a short to voltage each move the reported value is what separates a wiring fault from a real pressure problem.

    Source: ASE A9 Test Specification, Area F — three-wire reference/signal/ground sensor circuit diagnosisReport a problem with this question

Practice questions based on the ASE A9 Light Vehicle Diesel Engines task list. This site is not affiliated with or endorsed by the National Institute for Automotive Service Excellence (ASE). Injection pressures, torque specifications, clearances, glow-plug and heater resistance values, regeneration temperatures and service intervals are set by the engine manufacturer, differ by engine family and model year, and are revised by service bulletin — always work to the service information for the vehicle in front of you, never to a practice test. Emissions and aftertreatment hardware varies by model year and market. Diesel fuel systems operate at pressures that can inject fuel through skin; follow the manufacturer's depressurization and safety procedures. Confirm current test content and registration requirements with ASE before you test. About ASE certification →