ASE Light Vehicle Diesel Engines 2 — Questions and Answers
Question 1: In a common rail diesel injection system, what component is primarily responsible for generating the extremely high fuel pressure?
- High-pressure fuel pump (Correct answer)
- Lift pump
- Injector solenoid
- Fuel rail sensor
Correct answer: High-pressure fuel pump
The high-pressure pump pressurizes fuel and feeds the common rail, while the lift pump only supplies low-pressure fuel from the tank.
Question 2: A diesel engine has hard starting when cold but runs fine once warm. Which system should you inspect first?
- Glow plug system (Correct answer)
- EGR valve
- Turbocharger wastegate
- Fuel return line
Correct answer: Glow plug system
Glow plugs preheat the combustion chamber for cold starts, so a faulty glow plug system causes cold-start difficulty that disappears when warm.
Question 3: What is the purpose of the diesel particulate filter (DPF) regeneration process?
- Burn off accumulated soot (Correct answer)
- Cool the exhaust gas
- Increase fuel rail pressure
- Recharge the glow plugs
Correct answer: Burn off accumulated soot
Regeneration raises exhaust temperature to oxidize trapped soot into ash and gases, restoring DPF flow.
Question 4: Excessive black smoke from a diesel under load most commonly indicates which condition?
- Incomplete combustion from too much fuel or too little air (Correct answer)
- Coolant entering the cylinders
- Worn glow plugs
- Low battery voltage
Correct answer: Incomplete combustion from too much fuel or too little air
Black smoke results from unburned fuel, typically due to over-fueling, a restricted air intake, or a failing turbocharger.
Question 5: Which sensor signals the PCM about the position and speed of the crankshaft for diesel injection timing?
- Crankshaft position sensor (Correct answer)
- MAP sensor
- Coolant temperature sensor
- Oxygen sensor
Correct answer: Crankshaft position sensor
The crankshaft position sensor provides RPM and TDC reference data essential for precise injection timing.
Question 6: What does a glow plug 'wait-to-start' lamp indicate to the driver?
- Glow plugs are still preheating the cylinders (Correct answer)
- The DPF is regenerating
- Oil pressure is low
- The turbo is spooling
Correct answer: Glow plugs are still preheating the cylinders
The lamp stays on while the glow plugs heat the combustion chambers and goes out when it is safe to crank.
Question 7: Why is using clean, properly filtered fuel especially critical in modern high-pressure common rail diesels?
- Tight injector tolerances are easily damaged by contaminants (Correct answer)
- Fuel additives are illegal
- The lift pump cannot move dirty fuel
- Glow plugs require ultra-clean fuel
Correct answer: Tight injector tolerances are easily damaged by contaminants
Common rail injectors operate with micron-level clearances, so water or particulates can quickly cause wear and failure.
ASE Light Vehicle Diesel Engines 2
In a common rail diesel injection system, what component is primarily responsible for generating the extremely high fuel pressure?