ASE Practice Test (A9: Light Vehicle Diesel Engines) 3

Question 1

On a common rail diesel, which component maintains fuel pressure in the rail between injection events?

Accepted Answer

The fuel rail accumulator volume and pressure limiter valve

Answer

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?

Accepted Answer

Perform a cylinder leakage (leak-down) test and listen at the exhaust pipe

Answer

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?

Accepted Answer

Oxidize CO, hydrocarbons, and soluble organic fraction of exhaust

Answer

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?

Accepted Answer

Hard starting in cold weather with white smoke until warm

Answer

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?

Accepted Answer

Stuck or carbon-fouled vanes in the turbine housing

Answer

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.