Light Vehicle Diesel Engines Flashcards

A diesel engine has a rough idle and a knocking noise that worsens cold. Which is a likely cause?

Answer: Injector imbalance or incorrect injector calibration

An unbalanced or leaking injector delivers an incorrect fuel quantity, producing rough idle and combustion knock.

What is the function of a fuel-water separator on a diesel system?

Answer: Remove water from fuel before it reaches the injection system

The water separator traps water that would otherwise corrode and damage high-pressure injection components.

Which condition can trigger a forced or service DPF regeneration to be required?

Answer: High soot load from frequent short trips

Short trips prevent the exhaust from reaching regeneration temperature, allowing soot to accumulate until a service regen is needed.

White or grayish smoke at startup that smells of raw fuel usually indicates what?

Answer: Unburned fuel from poor cold combustion or weak glow plugs

White smoke of unburned fuel often points to incomplete cold combustion, such as failing glow plugs or low compression.

What should a technician do before opening a high-pressure common rail fuel system for service?

Answer: Relieve fuel pressure and observe safety procedures

Common rail pressure can exceed 30,000 psi, so pressure must be safely relieved to avoid high-pressure fuel injection injury.

Which sensor most directly informs the PCM of intake air mass for fuel and EGR control on a diesel?

Answer: Mass air flow (MAF) sensor

The MAF sensor measures incoming air mass, which the PCM uses to control fueling and EGR rates accurately.

Coolant in the engine oil (milky oil) on a diesel most commonly indicates which failure?

Answer: Head gasket or cooler leak allowing coolant into oil

A breached head gasket, cracked head, or failed oil cooler allows coolant to mix with engine oil, producing a milky appearance.