ASE AUTOMOBILE TECHNICIAN A9
LIGHT VEHICLE DIESEL ENGINES EXAM
QUESTION AND CORRECT ANSWERS
(VERIFIED ANSWERS) PLUS RATIONALES
2026 Q&A INSTANT DOWNLOAD PDF
1. A diesel engine has hard starting when cold. Which component most
directly affects cold starting?
A. EGR valve
B. Turbocharger
C. Glow plugs
D. Fuel return line
Rationale: Glow plugs heat the combustion chamber to aid ignition during
cold starts.
2. Excessive black smoke under acceleration is usually caused by:
A. Low compression
B. Overfueling or restricted air intake
C. Coolant contamination
D. Faulty glow plugs
Rationale: Black smoke indicates excess fuel or insufficient air for
combustion.
3. What is the primary purpose of a diesel particulate filter (DPF)?
A. Reduce NOx emissions
B. Trap and burn soot particles
C. Increase exhaust flow
D. Cool exhaust gases
Rationale: The DPF captures soot and periodically regenerates to burn it
off.
,4. A common rail diesel system primarily differs from older systems by:
A. Mechanical injectors
B. Lower fuel pressure
C. Electronically controlled high-pressure fuel delivery
D. No return fuel line
Rationale: Common rail systems use very high pressure and electronic
injector control.
5. Which sensor is most critical for determining injection timing?
A. MAP sensor
B. Crankshaft position sensor
C. Intake air temperature sensor
D. Fuel temperature sensor
Rationale: Injection timing is based on crankshaft position.
6. White smoke at idle on a warm diesel engine usually indicates:
A. Overfueling
B. Unburned fuel due to low combustion temperature
C. Oil burning
D. Restricted exhaust
Rationale: White smoke is often unburned fuel or coolant vapor.
7. What component controls exhaust gas recirculation flow?
A. PCM only
B. EGR valve
C. Turbo wastegate
D. DPF
Rationale: The EGR valve meters exhaust gases back into the intake.
8. Diesel knock is most often caused by:
A. Low cetane fuel
B. Advanced injection timing
C. High exhaust backpressure
D. Restricted fuel return
Rationale: Advanced timing causes rapid pressure rise and knock.
9. Which test best determines diesel engine compression?
A. Vacuum test
, B. Cylinder leak-down test
C. Compression test using a diesel gauge
D. Fuel pressure test
Rationale: Diesel engines require special high-range compression gauges.
10.What is the function of a high-pressure fuel pump?
A. Filter fuel
B. Increase fuel pressure for injection
C. Cool injectors
D. Regulate rail pressure electronically
Rationale: The pump mechanically creates high injection pressure.
11.A failed NOx sensor will most likely affect:
A. Fuel pressure
B. Glow plug operation
C. SCR/DEF system operation
D. Turbo boost
Rationale: NOx sensors monitor emissions for SCR control.
12.What happens during active DPF regeneration?
A. Exhaust is cooled
B. Exhaust temperature is increased to burn soot
C. Fuel injection is disabled
D. Turbo boost is reduced
Rationale: Regeneration burns soot by raising exhaust temperature.
13.A diesel engine with low power and excessive turbo lag may have:
A. Faulty injectors
B. Boost leak or failed turbocharger
C. High fuel pressure
D. Advanced timing
Rationale: Turbo or intake leaks reduce boost pressure.
14.Which component limits maximum turbocharger boost?
A. Intercooler
B. Wastegate or variable geometry control
LIGHT VEHICLE DIESEL ENGINES EXAM
QUESTION AND CORRECT ANSWERS
(VERIFIED ANSWERS) PLUS RATIONALES
2026 Q&A INSTANT DOWNLOAD PDF
1. A diesel engine has hard starting when cold. Which component most
directly affects cold starting?
A. EGR valve
B. Turbocharger
C. Glow plugs
D. Fuel return line
Rationale: Glow plugs heat the combustion chamber to aid ignition during
cold starts.
2. Excessive black smoke under acceleration is usually caused by:
A. Low compression
B. Overfueling or restricted air intake
C. Coolant contamination
D. Faulty glow plugs
Rationale: Black smoke indicates excess fuel or insufficient air for
combustion.
3. What is the primary purpose of a diesel particulate filter (DPF)?
A. Reduce NOx emissions
B. Trap and burn soot particles
C. Increase exhaust flow
D. Cool exhaust gases
Rationale: The DPF captures soot and periodically regenerates to burn it
off.
,4. A common rail diesel system primarily differs from older systems by:
A. Mechanical injectors
B. Lower fuel pressure
C. Electronically controlled high-pressure fuel delivery
D. No return fuel line
Rationale: Common rail systems use very high pressure and electronic
injector control.
5. Which sensor is most critical for determining injection timing?
A. MAP sensor
B. Crankshaft position sensor
C. Intake air temperature sensor
D. Fuel temperature sensor
Rationale: Injection timing is based on crankshaft position.
6. White smoke at idle on a warm diesel engine usually indicates:
A. Overfueling
B. Unburned fuel due to low combustion temperature
C. Oil burning
D. Restricted exhaust
Rationale: White smoke is often unburned fuel or coolant vapor.
7. What component controls exhaust gas recirculation flow?
A. PCM only
B. EGR valve
C. Turbo wastegate
D. DPF
Rationale: The EGR valve meters exhaust gases back into the intake.
8. Diesel knock is most often caused by:
A. Low cetane fuel
B. Advanced injection timing
C. High exhaust backpressure
D. Restricted fuel return
Rationale: Advanced timing causes rapid pressure rise and knock.
9. Which test best determines diesel engine compression?
A. Vacuum test
, B. Cylinder leak-down test
C. Compression test using a diesel gauge
D. Fuel pressure test
Rationale: Diesel engines require special high-range compression gauges.
10.What is the function of a high-pressure fuel pump?
A. Filter fuel
B. Increase fuel pressure for injection
C. Cool injectors
D. Regulate rail pressure electronically
Rationale: The pump mechanically creates high injection pressure.
11.A failed NOx sensor will most likely affect:
A. Fuel pressure
B. Glow plug operation
C. SCR/DEF system operation
D. Turbo boost
Rationale: NOx sensors monitor emissions for SCR control.
12.What happens during active DPF regeneration?
A. Exhaust is cooled
B. Exhaust temperature is increased to burn soot
C. Fuel injection is disabled
D. Turbo boost is reduced
Rationale: Regeneration burns soot by raising exhaust temperature.
13.A diesel engine with low power and excessive turbo lag may have:
A. Faulty injectors
B. Boost leak or failed turbocharger
C. High fuel pressure
D. Advanced timing
Rationale: Turbo or intake leaks reduce boost pressure.
14.Which component limits maximum turbocharger boost?
A. Intercooler
B. Wastegate or variable geometry control
