Examination Practice Questions And

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

• What is the primary function of a diesel engine turbocharger?
• Reduce engine emissions
• Increase air intake pressure
• Decrease fuel consumption
• Lower engine temperature

Rationale: A turbocharger compresses air entering the engine, allowing

more fuel to be burned efficiently and increasing power output.

• What type of fuel is used in a typical diesel engine?
• Gasoline
• Diesel fuel 1 / 4

• Ethanol
• Propane

Rationale: Diesel engines are designed to use diesel fuel, which has a higher

energy density and ignition temperature than gasoline.

• Which component is responsible for starting a diesel engine?
• Alternator
• Fuel injector
• Starter motor
• Glow plug

Rationale: The starter motor turns the engine over to initiate combustion,

while the glow plugs assist in cold starting.

• What is the purpose of a diesel engine glow plug?
• Pump fuel
• Reduce exhaust smoke
• Assist in cold starting
• Increase engine torque

Rationale: Glow plugs heat the combustion chamber to facilitate ignition in

cold conditions.

• Which system controls fuel delivery in modern diesel engines?
• Carburetor
• Distributor 2 / 4

• Fuel injection system
• Throttle body

Rationale: Fuel injection systems deliver precise fuel amounts at the correct

timing for efficient combustion.

• How often should diesel engine oil typically be changed under normal

operating conditions?

• Every 1,000 miles
• Every 3,000 miles
• Every 5,000–7,500 miles
• Every 15,000 miles

Rationale: Diesel engines require oil changes at intervals recommended by

manufacturers to maintain lubrication and engine life.

• What does the compression ratio in a diesel engine indicate?
• Air-fuel ratio
• The ratio of cylinder volume at bottom dead center to top dead

center

• Turbocharger efficiency
• Fuel injector pressure

Rationale: High compression in diesel engines ignites fuel without a spark

plug.

• Which type of diesel injection system uses high-pressure fuel delivered

directly into the combustion chamber? 3 / 4

• Indirect injection
• Port injection
• Direct injection
• Carbureted injection

Rationale: Direct injection sprays fuel straight into the cylinder, improving

efficiency and power.

• What is the main purpose of a diesel engine intercooler?
• Reduce fuel temperature
• Cool compressed air from the turbocharger
• Heat engine oil
• Increase exhaust flow

Rationale: Cooling the intake air increases its density, improving engine

efficiency and reducing combustion temperatures.

• Which of the following is a common cause of diesel engine white

smoke during startup?

• Worn piston rings
• Overheated engine
• Unburned fuel due to cold engine
• Air in the fuel system

Rationale: White smoke on cold starts usually indicates fuel is not fully

vaporizing in a cold combustion chamber.

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