NYC Auto Mechanic (Diesel) (DCAS)
Certification Practice Exam Questions With Correct Answers
Question 1: What is the primary difference between a diesel engine and a
gasoline engine?
Correct Answer: A diesel engine uses compression ignition (no spark plugs),
while a gasoline engine uses spark ignition.
Question 2: What is the purpose of glow plugs in a diesel engine?
Correct Answer: To pre-heat the combustion chamber to assist in cold
weather starting.
Question 3: What is the typical compression ratio range for a heavy-duty
diesel engine?
Correct Answer: 15:1 to 22:1.
Question 4: What happens if air enters the fuel system of a diesel engine?
Correct Answer: The engine will run rough, stall, or fail to start because the
fuel injectors cannot compress air to create the necessary injection pressure.
Question 5: What is the function of a turbocharger?
Correct Answer: To force more air into the cylinders, increasing engine
power and efficiency.
Question 6: What does the "cetane rating" of diesel fuel measure?
Correct Answer: The fuel's ignition quality and combustion speed (higher
cetane means easier/quieter cold starts).
Question 7: What causes white smoke from a diesel exhaust under normal
operating temperatures?
Correct Answer: Unburned fuel (often due to injector issues) or coolant
entering the combustion chamber (head gasket failure).
Question 8: What causes black smoke from a diesel exhaust?
Correct Answer: A rich fuel mixture (incomplete combustion), often caused
by a clogged air filter or injector issues.
Question 9: What causes blue smoke from a diesel exhaust?
Correct Answer: Burning engine oil (indicates worn piston rings, valve seals,
or cylinder liners).
Question 10: What is "blow-by"?
Correct Answer: Combustion gases leaking past the piston rings into the
crankcase.
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, Question 11: What is the function of the crankcase ventilation (CCV) system?
Correct Answer: To remove and route blow-by gases out of the crankcase to
prevent pressure buildup and oil contamination.
Question 12: What is a "wet sleeve" in a diesel engine block?
Correct Answer: A replaceable cylinder liner that is in direct contact with the
engine coolant.
Question 13: What is the primary function of the flywheel?
Correct Answer: To store rotational energy, smooth out power pulses, and
provide a friction surface for the clutch.
Question 14: What is the function of the harmonic balancer (vibration
damper)?
Correct Answer: To absorb and reduce torsional vibrations in the crankshaft.
Question 15: What is the main job of the top compression ring?
Correct Answer: To seal the combustion chamber and prevent blow-by.
Question 16: What is the main job of the oil control ring?
Correct Answer: To scrape excess oil off the cylinder walls and return it to
the crankcase.
Question 17: What are the four strokes of a diesel engine cycle?
Correct Answer: Intake, Compression, Power, Exhaust.
Question 18: What is the purpose of valve overlap?
Correct Answer: To improve the scavenging of exhaust gases and the intake
of fresh, cool air.
Question 19: What component converts the linear motion of the pistons into
rotational motion?
Correct Answer: The crankshaft.
Question 20: What component is responsible for opening and closing the
intake and exhaust valves?
Correct Answer: The camshaft.
Fuel Systems & Injection
Question 21: What is the function of the fuel transfer (lift) pump?
Correct Answer: To draw fuel from the tank and supply it to the high-
pressure injection pump at low pressure.
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Certification Practice Exam Questions With Correct Answers
Question 1: What is the primary difference between a diesel engine and a
gasoline engine?
Correct Answer: A diesel engine uses compression ignition (no spark plugs),
while a gasoline engine uses spark ignition.
Question 2: What is the purpose of glow plugs in a diesel engine?
Correct Answer: To pre-heat the combustion chamber to assist in cold
weather starting.
Question 3: What is the typical compression ratio range for a heavy-duty
diesel engine?
Correct Answer: 15:1 to 22:1.
Question 4: What happens if air enters the fuel system of a diesel engine?
Correct Answer: The engine will run rough, stall, or fail to start because the
fuel injectors cannot compress air to create the necessary injection pressure.
Question 5: What is the function of a turbocharger?
Correct Answer: To force more air into the cylinders, increasing engine
power and efficiency.
Question 6: What does the "cetane rating" of diesel fuel measure?
Correct Answer: The fuel's ignition quality and combustion speed (higher
cetane means easier/quieter cold starts).
Question 7: What causes white smoke from a diesel exhaust under normal
operating temperatures?
Correct Answer: Unburned fuel (often due to injector issues) or coolant
entering the combustion chamber (head gasket failure).
Question 8: What causes black smoke from a diesel exhaust?
Correct Answer: A rich fuel mixture (incomplete combustion), often caused
by a clogged air filter or injector issues.
Question 9: What causes blue smoke from a diesel exhaust?
Correct Answer: Burning engine oil (indicates worn piston rings, valve seals,
or cylinder liners).
Question 10: What is "blow-by"?
Correct Answer: Combustion gases leaking past the piston rings into the
crankcase.
Page 1 of 15
, Question 11: What is the function of the crankcase ventilation (CCV) system?
Correct Answer: To remove and route blow-by gases out of the crankcase to
prevent pressure buildup and oil contamination.
Question 12: What is a "wet sleeve" in a diesel engine block?
Correct Answer: A replaceable cylinder liner that is in direct contact with the
engine coolant.
Question 13: What is the primary function of the flywheel?
Correct Answer: To store rotational energy, smooth out power pulses, and
provide a friction surface for the clutch.
Question 14: What is the function of the harmonic balancer (vibration
damper)?
Correct Answer: To absorb and reduce torsional vibrations in the crankshaft.
Question 15: What is the main job of the top compression ring?
Correct Answer: To seal the combustion chamber and prevent blow-by.
Question 16: What is the main job of the oil control ring?
Correct Answer: To scrape excess oil off the cylinder walls and return it to
the crankcase.
Question 17: What are the four strokes of a diesel engine cycle?
Correct Answer: Intake, Compression, Power, Exhaust.
Question 18: What is the purpose of valve overlap?
Correct Answer: To improve the scavenging of exhaust gases and the intake
of fresh, cool air.
Question 19: What component converts the linear motion of the pistons into
rotational motion?
Correct Answer: The crankshaft.
Question 20: What component is responsible for opening and closing the
intake and exhaust valves?
Correct Answer: The camshaft.
Fuel Systems & Injection
Question 21: What is the function of the fuel transfer (lift) pump?
Correct Answer: To draw fuel from the tank and supply it to the high-
pressure injection pump at low pressure.
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