ASE Practice Test (A9: Light Vehicle Diesel Engines) 2 — Questions and Answers
Question 1: A light-duty diesel vehicle has a DTC for excessive diesel particulate filter (DPF) backpressure. What is the MOST likely cause?
- Faulty glow plugs
- Clogged DPF requiring regeneration or replacement (Correct answer)
- Failed EGR valve
- Leaking injector return line
Correct answer: Clogged DPF requiring regeneration or replacement
The DPF traps soot from diesel exhaust. Over time it becomes clogged and requires regeneration (burning off the soot at high temperatures). A DTC for excessive backpressure indicates the DPF is overloaded with particulate matter. If passive and active regenerations fail to clear it, the DPF may need forced regeneration or replacement.
Question 2: What is the function of the exhaust gas recirculation (EGR) system in a diesel engine?
- Increase engine power output
- Reduce NOx emissions by lowering combustion temperature (Correct answer)
- Increase fuel atomization in the combustion chamber
- Cool the turbocharger turbine wheel
Correct answer: Reduce NOx emissions by lowering combustion temperature
EGR reduces nitrogen oxides (NOx) emissions by recirculating a portion of exhaust gas back into the intake manifold. The inert exhaust gas dilutes the air-fuel mixture and absorbs heat, lowering peak combustion temperatures. Lower temperatures produce less NOx, which forms when nitrogen and oxygen combine at very high heat.
Question 3: During a diesel fuel system inspection, a technician finds air bubbles in the fuel return line. What should the technician check FIRST?
- Fuel injector spray pattern
- Fuel filter, supply lines, and lift pump for air leaks (Correct answer)
- High-pressure fuel pump delivery volume
- Injector return line pressure
Correct answer: Fuel filter, supply lines, and lift pump for air leaks
Air in diesel fuel is a critical problem that causes hard starting, rough running, and power loss. Air enters through the low-pressure (supply) side of the fuel system. The technician should start at the fuel tank and work toward the injection pump, checking the fuel filter housing, supply lines, lift pump, and fittings for leaks that allow air ingestion.
Question 4: A turbocharged diesel engine produces black smoke under acceleration. Which of the following is the MOST likely cause?
- Stuck open thermostat
- Restricted air intake or over-fueling condition (Correct answer)
- Low engine oil level
- Leaking head gasket
Correct answer: Restricted air intake or over-fueling condition
Black smoke in a diesel engine indicates incomplete combustion caused by an excessively rich mixture — too much fuel or too little air. A restricted air filter, collapsed intake duct, or a turbocharger not producing sufficient boost will limit air supply. A faulty injection pump or stuck injectors can also cause over-fueling. The air/fuel ratio must be balanced for complete combustion.
Question 5: What does the term "pilot injection" refer to in a common rail diesel fuel system?
- The first injection event per cycle, a small quantity before the main injection to reduce noise and combustion harshness (Correct answer)
- Injecting fuel into the intake manifold to pre-mix with air
- A safety injection that occurs only during cold starts
- The injection pulse that triggers the glow plug circuit
Correct answer: The first injection event per cycle, a small quantity before the main injection to reduce noise and combustion harshness
Common rail systems can perform multiple injection events per cycle. Pilot injection delivers a very small amount of fuel just before the main injection. This small pilot charge begins combustion and raises cylinder temperature and pressure slightly, so the main injection ignites more smoothly and gradually. This reduces the characteristic diesel knock and combustion noise.
Question 6: A diesel engine has low power and a DTC indicating low rail pressure. The high-pressure pump is functioning normally. What should the technician check next?
- Glow plug resistance
- Injector return (leak-off) volume to check for worn injector tips (Correct answer)
- EGR valve position sensor
- Diesel oxidation catalyst efficiency
Correct answer: Injector return (leak-off) volume to check for worn injector tips
If the high-pressure pump is working correctly but rail pressure is still low, fuel is leaking back somewhere in the high-pressure circuit. The most common cause is worn fuel injectors allowing excessive fuel to bypass back through the return (leak-off) line. A leak-off test measures return volume from each injector; excessive return indicates worn injectors that cannot hold rail pressure.
ASE Practice Test (A9: Light Vehicle Diesel Engines) 2
A light-duty diesel vehicle has a DTC for excessive diesel particulate filter (DPF) backpressure.
What is the MOST likely cause?