ASE - Automotive Service Excellence Practice Test (A9: Light Vehicle Diesel Engines) 1 — Questions and Answers
Question 1: A technician is diagnosing a light-duty diesel engine that cranks but will not start in cold weather. The glow plug indicator light does not illuminate. What is the MOST likely cause?
- Faulty crankshaft position sensor
- Failed glow plug control module (Correct answer)
- Plugged fuel filter
- Low compression in cylinder #1
Correct answer: Failed glow plug control module
The glow plug control module energizes the glow plugs and signals the dashboard indicator. If the module fails, neither the glow plugs nor the lamp will activate, making cold starts difficult or impossible.
Question 2: A common-rail diesel fuel system is operating at lower than specified rail pressure at idle. Which component should the technician inspect FIRST?
- Fuel pressure regulator/inlet metering valve (Correct answer)
- High-pressure fuel injectors
- Low-pressure lift pump
- Diesel particulate filter differential pressure sensor
Correct answer: Fuel pressure regulator/inlet metering valve
The inlet metering valve (or fuel pressure regulator) on the high-pressure pump controls the amount of fuel entering the pump, directly determining rail pressure. A sticking or failed regulator is the most common cause of low rail pressure at idle.
Question 3: White smoke from a diesel engine that persists after the engine reaches normal operating temperature is MOST likely caused by:
- An overly rich air-fuel mixture
- Incomplete combustion due to coolant entering the combustion chamber (Correct answer)
- Excessive lubricating oil consumption
- A clogged diesel oxidation catalyst (DOC)
Correct answer: Incomplete combustion due to coolant entering the combustion chamber
Persistent white smoke after warm-up typically indicates coolant (water vapor) entering the combustion chamber through a blown head gasket or cracked cylinder head. Cold-start white smoke clears once the engine warms, but coolant intrusion continues indefinitely.
Question 4: A diesel technician performs a cylinder contribution (cut-out) test and finds that dropping cylinder #3 produces no change in engine roughness or RPM. What does this indicate?
- Cylinder #3 is over-fueling compared to other cylinders
- Cylinder #3 injector is not contributing power to the engine (Correct answer)
- The high-pressure fuel pump is weak
- Cylinder #3 has excessive compression
Correct answer: Cylinder #3 injector is not contributing power to the engine
During a cylinder contribution test, a normally firing cylinder will cause a noticeable RPM drop when cut out. If cutting out a cylinder causes no change, that cylinder is not producing power, indicating a failed injector, no compression, or no fuel delivery to that cylinder.
Question 5: After replacing a diesel high-pressure fuel injector, the technician must perform which of the following procedures using a scan tool before returning the vehicle to service?
- Injector quantity/trim code programming (Correct answer)
- Throttle position sensor relearn
- Diesel oxidation catalyst regeneration
- Crankshaft reluctor wheel relearn
Correct answer: Injector quantity/trim code programming
Modern common-rail injectors have unique flow-rate calibration codes (IMA, C2I, or QR codes) that must be programmed into the ECM so the engine management system can compensate for individual injector flow characteristics and maintain correct fueling balance.
Question 6: A diesel vehicle equipped with a selective catalytic reduction (SCR) system has a MIL on and a DTC for low NOx conversion efficiency. Which component is MOST likely at fault?
- Diesel particulate filter (DPF) — full of soot
- Diesel exhaust fluid (DEF) dosing injector — clogged or failed (Correct answer)
- Exhaust gas recirculation (EGR) valve — stuck open
- Turbocharger wastegate — stuck closed
Correct answer: Diesel exhaust fluid (DEF) dosing injector — clogged or failed
The SCR system reduces NOx by injecting diesel exhaust fluid (DEF/urea) into the exhaust stream ahead of the SCR catalyst. A clogged or failed DEF dosing injector prevents urea from reaching the catalyst, drastically reducing NOx conversion efficiency.
ASE - Automotive Service Excellence Practice Test (A9: Light Vehicle Diesel Engines) 1
A technician is diagnosing a light-duty diesel engine that cranks but will not start in cold weather.
The glow plug indicator light does not illuminate.
What is the MOST likely cause?