Exam Questions And Correct Answers

Diesel Mechanic Certification Practice Exam Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• What is the primary function of the turbocharger in a diesel engine?
• To increase engine oil pressure
• To increase air intake pressure and improve combustion efficiency
• To reduce exhaust emissions only
• To regulate coolant temperature

Rationale: The turbocharger compresses intake air, allowing more oxygen

into the combustion chamber for better fuel efficiency and power output.

• What does the acronym EGR stand for in diesel technology?
• Engine Gas Reduction
• Exhaust Gas Recirculation
• Emission Gas Regulation
• Exhaust Grade Retarder 1 / 4

Rationale: EGR recirculates a portion of exhaust gas into the intake air to

reduce NOx emissions.

• What is the purpose of the fuel lift pump in a diesel engine?
• To transfer fuel from the tank to the injection pump
• To cool the fuel system
• To mix air with fuel before combustion
• To increase fuel pressure to injection levels

Rationale: The lift pump moves fuel from the tank to the injection system

under low pressure.

• Which component is responsible for controlling injector timing in a

common rail diesel system?

• Fuel pump
• Glow plug
• Engine Control Module (ECM)
• Crankshaft position sensor

Rationale: The ECM calculates and controls injection timing based on

sensor data for precision combustion.

• What type of combustion occurs in a diesel engine?
• Spark ignition 2 / 4

• Compression ignition
• Detonation
• Thermal ignition

Rationale: Diesel engines use compression to ignite fuel rather than a

spark.

• Which of the following is a symptom of a clogged fuel filter?
• Increased turbo boost
• Smooth idle
• Hard starting and power loss
• Increased fuel economy

Rationale: A restricted fuel filter limits flow, causing power loss and

starting issues.

• What is the main function of glow plugs?
• To provide spark for ignition
• To preheat air in the combustion chamber for easier cold starts
• To reduce emissions
• To increase boost pressure

Rationale: Glow plugs warm the combustion area, aiding fuel ignition in

cold conditions.

• / 4

• Diesel fuel’s cetane number measures what?
• Ignition quality of the fuel
• Energy content
• Lubrication capability
• Density

Rationale: Cetane number indicates how quickly and efficiently fuel ignites

under compression.

• Which tool measures cylinder compression in a diesel engine?
• Torque wrench
• Compression tester
• Vacuum gauge
• Feeler gauge

Rationale: A compression tester determines if cylinder pressure is within

specifications.

• What causes black smoke from the exhaust of a diesel engine?
• Coolant leak
• Incomplete combustion due to excess fuel
• Low oil pressure
• Exhaust leak

Rationale: Black smoke indicates too much fuel or insufficient air, leading

to incomplete combustion.

• / 4