Exam Questions And Correct Answers

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

• Which of the following best describes the function of an EGR (Exhaust Gas

Recirculation) valve?

• Reduces noise from exhaust gases
• Recirculates exhaust gases to reduce NOx emissions
• Increases turbocharger boost pressure
• Controls fuel injection timing

Rationale: EGR reduces nitrogen oxides by lowering combustion

temperature through exhaust gas recirculation.

• When diagnosing a diesel with excessive black smoke, which condition is

most likely?

• Over-fueling or restricted air intake 1 / 4

• Low compression
• Faulty injector timing
• Coolant leak

Rationale: Black smoke indicates incomplete combustion due to too much

fuel or too little air.

• What is the primary purpose of a diesel particulate filter (DPF)?
• Reduce NOx
• Capture and store soot from exhaust gases
• Cool the exhaust
• Control fuel delivery

Rationale: The DPF traps soot to meet emission standards and periodically

regenerates to burn it off.

• A diesel engine exhibits hard starting in cold weather. What component is

most likely faulty?

• Glow plugs
• EGR valve
• Injector seals
• Turbocharger

Rationale: Glow plugs preheat air in the combustion chamber for better

cold starts. 2 / 4

• What does the term “common rail” refer to in diesel fuel systems?
• A shared high-pressure fuel line supplying injectors
• A single return line
• The intake manifold
• An exhaust manifold

Rationale: Common rail systems deliver fuel under high pressure from one

rail to each injector electronically.

6. The cetane number of diesel fuel measures:

• Ignition quality of the fuel
• Energy content per gallon
• Sulfur content
• Lubrication capacity

Rationale: Higher cetane improves ignition delay and smoothness of

combustion.

• What type of exhaust smoke usually indicates oil burning?
• Black
• Gray
• Blue 3 / 4

• White

Rationale: Blue smoke signals oil entering the combustion chamber.

• Which component controls boost pressure in a turbocharged diesel?
• Wastegate or variable geometry actuator
• Throttle body
• Intercooler
• Fuel pressure regulator

Rationale: The wastegate or actuator regulates turbine speed to control

boost.

• What causes diesel “runaway”?
• Excess fuel injection
• Engine burning its own oil
• Faulty EGR valve
• Stuck injector solenoid

Rationale: Oil entering intake allows uncontrolled combustion even with

fuel shut off.

• Which type of diesel injector uses solenoids for precise fuel metering?
• Electronic unit injector (EUI)
• Mechanical pump-line-nozzle
• / 4