Five free practice questions on hybrid engine diagnosis: content area B of the ASE L3 Light Duty Hybrid/Electric Vehicle Specialist test, 6 of its 45 scored questions (13.3%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.
01The questions
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Many plug-in hybrids keep the fuel tank sealed off from the charcoal canister by an electrically operated isolation valve, opening it mainly when the driver asks to refuel. Why is the tank sealed?
- The engine runs too seldom to purge the canister
- To keep fuel vapor away from the HV battery area
- To hold positive pressure at the fuel pump inlet
- To let the tank be filled faster at the gas station
Show the answer
A. Correct. A canister is cleaned only when a running engine draws purge flow through it, and a plug-in hybrid may go days without running its engine. Sealing the tank keeps daily tank vapor out of the canister, so it holds mainly refueling vapor and does not overload and vent to the air.
Why not the others
B. The isolation valve sits in the vapor line between the tank and the canister and controls when tank vapor may reach the canister; it has nothing to do with the battery's location. It is there because the engine seldom runs to purge the canister.
C. The fuel pump draws liquid fuel from the bottom of the tank and does not rely on vapor pressure at its inlet. The tank is sealed to keep vapor out of a canister that the seldom-running engine cannot purge often.
D. Sealing makes refueling slower, not faster: the system must first release tank pressure into the canister before fuel can be added. The reason for the seal is the engine's limited running time to purge the canister.
ID L3-B-001 · Task L3-B-11 (ASE task list)
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A hybrid's engine uses late intake valve closing (Atkinson cycle). A cranking compression test by the manufacturer's procedure reads 128, 131, 124 and 130 psi on cylinders 1 through 4. The manufacturer specifies a minimum of 102 psi and no more than 15 psi difference between cylinders. What do these results show?
- All four cylinders are low from worn rings
- Every cylinder meets the specification
- Cylinder 3 is below the minimum
- Cylinder 3 exceeds the allowed difference
Show the answer
B. Correct. Every reading is above 102 psi and the spread is 131 - 124 = 7 psi, within 15 psi. Late intake valve closing pushes part of each cylinder's charge back into the intake, so cranking compression is lower than the compression ratio suggests, which is why the manufacturer's own figures are used.
Why not the others
A. The readings look low against an engine without late intake closing, but the manufacturer's figures already allow for it: every cylinder is above the 102 psi minimum. Judge against this engine's specification, not against memory of other engines.
C. Cylinder 3 reads 124 psi, which is 22 psi above the 102 psi minimum. It passes.
D. The spread is 131 - 124 = 7 psi, under the 15 psi allowed. Cylinder 3 is the lowest reading but within the limit.
ID L3-B-009 · Task L3-B-4 (ASE task list)
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Why does the manufacturer's procedure for an idle tailpipe emissions test on a hybrid call for its maintenance (inspection) mode?
- So the HV battery is isolated for the test
- So the engine runs open loop for the test
- So the MIL and DTC logic are suspended
- So the engine keeps running throughout
Show the answer
D. Correct. Normally the hybrid system stops the engine whenever it is not needed, so it could shut off partway through the measurement. The maintenance mode holds the engine running so the gas analysis or idle check can be completed.
Why not the others
A. The mode does not open the HV system; the hybrid system stays in READY TO DRIVE. Its job is to keep the engine running so the test is not interrupted.
B. An emissions test measures the engine as it normally operates, in closed loop once warm; forcing open loop would distort the result. The mode keeps the engine running instead of letting the system stop it.
C. The mode is not there to switch off the MIL or the DTC logic; a tailpipe test measures the exhaust, not the fault memory. Its purpose is to keep the engine from stopping during the measurement.
ID L3-B-011 · Task L3-B-5 (ASE task list)
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A hybrid's warm engine is held at idle by the manufacturer's maintenance mode. Any of these can be done in this mode EXCEPT:
- a cranking compression test.
- checking fuel trims at warm idle.
- measuring tailpipe emissions at idle.
- listening for an exhaust leak at idle.
Show the answer
A. Correct. A cranking compression test needs the engine turned without fuel injected, not running. The maintenance mode keeps the engine fueled and running, so the scan tool's separate cranking test is used instead.
Why not the others
B. This can be done: with the engine held running and warm, the scan tool shows closed-loop fuel trims at idle.
C. This can be done: the mode exists to keep the engine running so a steady idle exhaust measurement is possible.
D. This can be done: an exhaust leak is heard or found with the engine running, and the mode keeps it running long enough to search.
ID L3-B-012 · Task L3-B-5 (ASE task list)
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A hybrid's motor/generator cranks the engine at its normal cranking speed, but the engine does not start. Any of these could be the cause EXCEPT:
- an open fuel pump relay circuit.
- a broken timing chain.
- a failed 12 V starter motor.
- a blown fuse feeding all the ignition coils.
Show the answer
C. Correct. On this vehicle the motor/generator is doing the cranking and the engine turns at its normal speed, so no starter fault can explain the no-start. Look at fuel, spark, compression and valve timing instead.
Why not the others
A. This could be the cause: with the pump relay open, no fuel reaches the injectors, so the engine turns but cannot start.
B. This could be the cause: with the chain broken the camshafts stop, the valves no longer open in time, and the engine cannot run even though it turns.
D. This could be the cause: with no power to any coil there is no spark, so the engine turns at normal speed but cannot start.
ID L3-B-014 · Task L3-B-7 (ASE task list)
02Answer key
| Question | Answer |
|---|---|
| Q1 | A |
| Q2 | B |
| Q3 | D |
| Q4 | A |
| Q5 | C |
03What area B covers
This content area covers the gasoline engine in a hybrid: telling whether the system is cranking it or it is running on its own, sorting engine faults from hybrid drive faults, compression and vacuum readings, keeping the engine running for service and emissions checks, no-crank and no-start faults, start/stop operation, cooling system service, hybrid EVAP systems, and the needs of an engine that runs only part of the time.
The ASE test specification calls this content area “Internal Combustion Engine”. It carries 6 of the 45 scored questions, 13.3% of the test.
- Read engine DTCs alongside hybrid system DTCs
- Tell cranking from running in scan data
- Separate engine faults from hybrid drive faults
- Run cranking compression and manifold vacuum tests
- Hold the engine running for service and emissions tests
- Diagnose no-crank, no-start and auto stop faults
- Service the cooling system and hybrid EVAP parts
- Apply maintenance rules for a seldom-run engine
04What the questions turn on
Fix the engine code first
When an engine DTC sits beside a hybrid DTC saying engine power is below the request, the hybrid code reports an effect and the engine code points at a cause. The engine DTC is repaired first, then both are cleared and the hybrid DTC rechecked.
Maintenance mode keeps it running
The hybrid system normally stops the engine whenever it is not needed, so an idle emissions check uses the manufacturer's maintenance mode to hold it running. A cranking compression test needs the engine turned without fuel, so it uses the scan tool's separate cranking test.
Judge compression by the maker's figures
An engine with late intake valve closing pushes part of each charge back into the intake, so its cranking compression is lower than its compression ratio suggests. Readings are judged against the manufacturer's own minimum and its allowed spread between cylinders.
A silent engine can still start
In READY TO DRIVE the system starts and stops the engine on its own, for charge, warm-up or other needs. Before hands go near the drive belt or cooling fan, the vehicle is switched OFF and the smart key kept away as the manufacturer directs.
05The other content areas
The L3 test has five content areas. Each has its own page of free questions:
The ASE L3 Light Duty Hybrid/Electric Vehicle Specialist practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.
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