ASE Practice Test (A9: Light Vehicle Diesel Engines) 3 — Questions and Answers
Question 1: On a common rail diesel, which component maintains fuel pressure in the rail between injection events?
- Pressure relief valve only
- High-pressure fuel pump combined with the pressure limiter valve
- The fuel rail accumulator volume and pressure limiter valve (Correct answer)
- The lift pump working at high pressure
Correct answer: The fuel rail accumulator volume and pressure limiter valve
The fuel rail (accumulator) stores high-pressure fuel between injection events. Its volume dampens pressure pulsations from the high-pressure pump and individual injections. A pressure limiter valve (pressure relief valve) opens if pressure exceeds a safe maximum. Together they maintain stable rail pressure for precise injection control.
Question 2: A technician is performing a compression test on a diesel engine and finds one cylinder significantly lower than the others. Which test would BEST confirm a burned exhaust valve?
- Perform a cylinder leakage (leak-down) test and listen at the exhaust pipe (Correct answer)
- Check fuel injector pop pressure on that cylinder
- Measure glow plug resistance on that cylinder
- Perform a running compression test at idle only
Correct answer: Perform a cylinder leakage (leak-down) test and listen at the exhaust pipe
A cylinder leakage test pressurizes the cylinder and measures the percentage of air that escapes. If air is heard escaping at the exhaust pipe (tailpipe or muffler), a burned or leaking exhaust valve is indicated. Air escaping at the intake suggests an intake valve problem. Bubbles in coolant suggest a head gasket leak.
Question 3: What is the purpose of the diesel oxidation catalyst (DOC) in the aftertreatment system?
- Trap and store particulate matter (soot)
- Oxidize CO, hydrocarbons, and soluble organic fraction of exhaust (Correct answer)
- Reduce NOx through selective catalytic reduction
- Increase exhaust backpressure for engine braking
Correct answer: Oxidize CO, hydrocarbons, and soluble organic fraction of exhaust
The diesel oxidation catalyst (DOC) uses a platinum or palladium catalyst to oxidize harmful exhaust components including carbon monoxide (CO), unburned hydrocarbons (HC), and the soluble organic fraction of particulate matter. The DOC also generates heat during regeneration events to help the downstream DPF reach soot-burning temperatures.
Question 4: Which diesel engine complaint is MOST associated with faulty or clogged glow plugs?
- Hard starting in cold weather with white smoke until warm (Correct answer)
- Black smoke under load at operating temperature
- Fuel in coolant
- Excessive oil consumption
Correct answer: Hard starting in cold weather with white smoke until warm
Glow plugs preheat the prechamber or combustion chamber to aid cold starting. Faulty or clogged glow plugs lead to hard cold starts because the cylinder air temperature is insufficient to ignite the diesel fuel by compression alone at low temperatures. White smoke (unburned fuel) is common until the engine warms up. At operating temperature, glow plugs are not active.
Question 5: A technician finds that a variable geometry turbocharger (VGT) is not responding to commands. A scan tool shows no DTCs. What is the MOST likely cause?
- Worn turbocharger bearings
- Stuck or carbon-fouled vanes in the turbine housing (Correct answer)
- Faulty boost pressure sensor
- Cracked exhaust manifold
Correct answer: Stuck or carbon-fouled vanes in the turbine housing
VGT turbochargers use moveable vanes to control exhaust flow and boost. Carbon deposits and soot accumulation from diesel exhaust frequently cause the vanes to stick or move sluggishly. If no DTCs are present, the actuator may be commanding movement but the vanes are physically stuck. Cleaning the vanes or replacing the turbocharger is the typical repair.
Question 6: What is the primary purpose of the water separator found in the diesel fuel system?
- Remove sulfur from diesel fuel
- Separate and drain water from diesel fuel before it reaches the injection system (Correct answer)
- Filter large particulate contamination from fuel
- Reduce fuel temperature before injection
Correct answer: Separate and drain water from diesel fuel before it reaches the injection system
Water contamination in diesel fuel is extremely damaging to precision injection components. The water separator (often integrated with the fuel filter) removes water that has condensed in the fuel tank or entered through contamination. Many vehicles have a water-in-fuel (WIF) sensor and a drain valve at the bottom of the separator. Regular draining is essential maintenance.
ASE Practice Test (A9: Light Vehicle Diesel Engines) 3
On a common rail diesel, which component maintains fuel pressure in the rail between injection events?