ASE Test Electronic Diesel Engine Diagnosis 2 — Questions and Answers
Question 1: A diesel engine cranks but will not start, and a scan tool shows zero RPM signal during cranking. Which sensor is the most likely cause?
- Crankshaft position sensor (Correct answer)
- Coolant temperature sensor
- Barometric pressure sensor
- Fuel rail pressure sensor
Correct answer: Crankshaft position sensor
The crankshaft position sensor provides the RPM signal the ECM needs to time injection, so a no-RPM-during-cranking reading points to it.
Question 2: When diagnosing a high-pressure common rail (HPCR) system, what is the safest first step before disconnecting any line?
- Relieve fuel rail pressure and let the engine cool (Correct answer)
- Crank the engine to purge fuel
- Disconnect the battery only
- Loosen the fuel cap and start the engine
Correct answer: Relieve fuel rail pressure and let the engine cool
Common rail systems hold extremely high pressure that can cause injection injury, so rail pressure must be relieved and the engine allowed to cool first.
Question 3: A technician finds excessive injector return (back-leakage) fuel on a common rail engine. What does this typically indicate?
- Worn or leaking injectors (Correct answer)
- A clogged air filter
- A faulty glow plug relay
- Low coolant level
Correct answer: Worn or leaking injectors
Excessive return flow from one or more injectors indicates internal injector wear or leakage that bleeds off rail pressure.
Question 4: Which diagnostic tool is used to measure individual cylinder contribution on an electronically controlled diesel?
- Cylinder balance / contribution test via scan tool (Correct answer)
- A timing light
- A compression-only gauge
- A vacuum gauge
Correct answer: Cylinder balance / contribution test via scan tool
A scan tool cylinder balance (contribution) test cuts injectors individually to reveal weak cylinders.
Question 5: A glow plug warning lamp stays on and the engine is hard to start cold. What should be checked first?
- Glow plug circuit and relay operation (Correct answer)
- Engine oil viscosity
- Transmission fluid level
- Exhaust back pressure
Correct answer: Glow plug circuit and relay operation
A glow plug lamp fault with hard cold starts points directly to the glow plug circuit, relay, or individual plugs.
Question 6: What does a fuel rail pressure that fails to reach commanded value during cranking most likely indicate?
- A weak transfer pump, restriction, or failing high-pressure pump (Correct answer)
- A bad oxygen sensor
- An open thermostat
- A loose gas cap
Correct answer: A weak transfer pump, restriction, or failing high-pressure pump
Inability to build commanded rail pressure usually stems from a supply restriction, a weak lift pump, or a failing high-pressure pump.
Question 7: Before condemning an electronic diesel injector, a technician should first verify what?
- Correct fuel supply, return, and rail pressure with no air in the system (Correct answer)
- Tire pressure
- Coolant color
- Serpentine belt tension
Correct answer: Correct fuel supply, return, and rail pressure with no air in the system
Adequate fuel supply, sealed return, proper rail pressure, and an air-free system must be confirmed before blaming an injector.
ASE Test Electronic Diesel Engine Diagnosis 2
A diesel engine cranks but will not start, and a scan tool shows zero RPM signal during cranking.
Which sensor is the most likely cause?