ASE Light Vehicle Diesel Engines 3 — Questions and Answers
Question 1: What is the primary function of the exhaust gas recirculation (EGR) system on a diesel engine?
- Reduce NOx emissions by lowering combustion temperature (Correct answer)
- Increase fuel pressure
- Preheat the intake air
- Filter soot from the exhaust
Correct answer: Reduce NOx emissions by lowering combustion temperature
EGR routes inert exhaust gas back into the intake to lower peak combustion temperatures and reduce nitrogen oxide formation.
Question 2: A turbocharged diesel shows low boost and sluggish acceleration. Which component should you check for a stuck or leaking condition?
- Turbocharger and its boost control/wastegate (Correct answer)
- Glow plug relay
- DPF pressure sensor
- Crankshaft sensor
Correct answer: Turbocharger and its boost control/wastegate
Low boost is commonly caused by a failing turbo, leaking charge pipes, or a malfunctioning wastegate/VGT actuator.
Question 3: What fluid is injected into the exhaust stream in an SCR (selective catalytic reduction) system?
- Diesel exhaust fluid (urea/DEF) (Correct answer)
- Engine coolant
- Diesel fuel
- Brake fluid
Correct answer: Diesel exhaust fluid (urea/DEF)
DEF (a urea-water solution) reacts in the SCR catalyst to convert NOx into nitrogen and water.
Question 4: On a diesel engine, what is the most likely cause of a 'no-start' with normal cranking speed and no smoke?
- No fuel delivery or loss of fuel pressure (Correct answer)
- Worn piston rings
- Faulty cabin heater
- Low tire pressure
Correct answer: No fuel delivery or loss of fuel pressure
If the engine cranks normally but never fires and produces no smoke, fuel is likely not reaching the cylinders.
Question 5: Which measurement is most useful for diagnosing a diesel cylinder with low power contribution?
- Compression test (Correct answer)
- Battery load test
- Tire tread depth
- Coolant pH
Correct answer: Compression test
A compression test reveals worn rings, valves, or head gasket issues that reduce a cylinder's combustion sealing.
Question 6: What does a clogged or saturated fuel filter typically cause in a diesel engine?
- Power loss and stalling under load (Correct answer)
- Excessive idle speed
- Glow plug overheating
- Increased boost pressure
Correct answer: Power loss and stalling under load
A restricted fuel filter starves the engine of fuel, causing hesitation, power loss, and stalling especially under heavy load.
Question 7: Why do diesel engines compress air to a much higher ratio than gasoline engines?
- Heat of compression ignites the fuel without a spark (Correct answer)
- To reduce exhaust noise
- To power the alternator
- To cool the intake charge
Correct answer: Heat of compression ignites the fuel without a spark
Diesels rely on compression-ignition, where high compression heats the air enough to ignite injected fuel without a spark plug.
ASE Light Vehicle Diesel Engines 3
What is the primary function of the exhaust gas recirculation (EGR) system on a diesel engine?