Ohio Diesel Mechanic Practice Exam

Ohio Diesel Mechanic Practice Exam Questions And Correct Answers (Verified Answers) Plus Rationales 2025|2026 Q&A | Instant Download Pdf

• What is the primary function of a diesel engine’s turbocharger?
• Increase fuel efficiency by reducing fuel flow
• Increase air intake pressure to improve combustion
• Reduce engine speed during idle
• Cool the exhaust gases

Rationale: The turbocharger compresses incoming air, allowing more

oxygen for combustion and boosting engine power.

• What is the main difference between diesel and gasoline engines?
• Diesel engines use spark plugs
• Gasoline engines use compression ignition
• Diesel engines use compression ignition instead of spark ignition
• Gasoline engines use fuel injectors only 1 / 4

Rationale: Diesel engines ignite fuel by heat from compression rather

than spark ignition.

• What type of fuel system is most commonly used in modern diesel

engines?

• Carburetor
• Common rail direct injection
• Rotary pump
• Mechanical injector system

Rationale: Common rail systems allow high-pressure fuel delivery

with precise electronic control for efficiency and emissions.

• What component removes soot and particulates from diesel exhaust?
• Catalytic converter
• Diesel particulate filter (DPF)
• EGR valve
• Turbocharger

Rationale: The DPF traps soot particles to prevent them from

entering the atmosphere.

• What is the purpose of the EGR (Exhaust Gas Recirculation) valve?
• To increase fuel flow
• To improve oil circulation
• To reduce NOx emissions by recirculating exhaust gas
• To increase engine compression 2 / 4

Rationale: EGR lowers combustion temperature, reducing nitrogen

oxide emissions.

6. The term "blow-by" refers to:

• Combustion gases escaping past piston rings into the crankcase
• Air entering the intake manifold
• Oil leaking into the exhaust
• Coolant entering the cylinders

Rationale: Blow-by gases result from imperfect sealing between

piston rings and cylinder walls.

• What does the intercooler do in a turbocharged diesel engine?
• Increases exhaust temperature
• Cools the compressed air from the turbo before it enters the

intake

• Cools the engine coolant
• Mixes fuel and air

Rationale: Cooling the air increases its density, improving engine

performance and efficiency.

8. Diesel fuel cetane rating indicates:

• The amount of sulfur in the fuel
• The ignition quality of the fuel
• The octane number equivalent
• The lubrication level 3 / 4

Rationale: Cetane number measures how quickly diesel fuel ignites

after injection.

• What happens when fuel injectors are leaking?
• Reduced fuel economy
• Hard starting and black smoke
• High oil pressure
• Engine knock stops

Rationale: Leaky injectors cause excessive fuel in cylinders, leading to

rich mixtures and smoke.

• What instrument measures cylinder compression?
• Vacuum gauge
• Compression gauge
• Fuel pressure tester
• Dial indicator

Rationale: A compression gauge measures the maximum pressure

developed during the compression stroke.

• Diesel engines generally have higher compression ratios

because:

• They run at lower temperatures
• They rely on compression heat to ignite fuel
• They use spark plugs
• They burn less fuel
• / 4