Practice Exam Questions And Correct

ASE A9 Light Vehicle Diesel Engines Practice Exam Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• Which of the following is a common symptom of low compression in a

diesel engine?

• Excessive oil consumption
• Hard starting
• Overheating
• Engine knocking

Low compression reduces the heat generated during compression, making it harder for the air-fuel mixture to ignite, resulting in hard starting.

• What is the purpose of a diesel engine glow plug?
• Increase engine compression
• Reduce fuel consumption 1 / 4

• Aid cold starting
• Control exhaust emissions

Glow plugs preheat the combustion chamber, helping ignite fuel in cold conditions.

• Which component directly controls fuel injection timing in most

modern diesel engines?

• Crankshaft
• ECU (Electronic Control Unit)
• Camshaft
• Turbocharger

The ECU adjusts injection timing based on sensors and engine operating conditions.

• What is the main function of the diesel engine intercooler?
• Reduce oil temperature
• Cool compressed intake air
• Increase exhaust pressure
• Filter fuel

Cooling the compressed air increases its density, improving combustion efficiency and reducing engine knock.

• Which type of fuel injection system uses a high-pressure pump to

deliver fuel to each injector individually?

• Unit injector 2 / 4

• Common rail
• Distributor-type inline pump
• Carbureted system

A distributor-type inline pump routes high-pressure fuel to each injector sequentially.

• What is a common cause of white smoke from a diesel engine at

startup?

• Unburned fuel due to cold engine
• Overheating
• Oil burning
• EGR malfunction

White smoke is usually unburned fuel that fails to ignite because the combustion chamber is too cold.

7. Excessive black smoke in a diesel engine generally indicates:

• Lean fuel mixture
• Coolant leak
• Overfueling or incomplete combustion
• Exhaust restriction

Black smoke is caused by too much fuel or poor atomization, resulting in incomplete combustion.

• A diesel engine’s turbocharger fails to boost. What is the first likely

cause? 3 / 4

• Low engine compression
• Restricted air intake or exhaust
• Faulty glow plugs
• Weak battery

A restriction in intake or exhaust prevents proper turbo operation.

• What is the main purpose of the EGR (Exhaust Gas Recirculation)

system in diesel engines?

• Increase power
• Reduce oil consumption
• Improve fuel economy
• Reduce NOx emissions

EGR recirculates exhaust gas to lower combustion temperatures, reducing NOx formation.

10. Diesel fuel injector nozzles are designed to:

• Increase fuel volume
• Reduce compression
• Atomize fuel for proper combustion
• Heat the fuel

Proper atomization ensures efficient combustion and reduces smoke.

• What is a common symptom of a failing diesel fuel lift pump?
• Engine overheating
• Hard starting and stalling
• / 4