Practice Exam Questions And Correct

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

• What is the primary function of a marine diesel engine’s raw water

pump?

• To circulate coolant through the closed-loop system
• To supply seawater for engine cooling
• To maintain oil pressure in the crankcase
• To provide air for combustion

Rationale: The raw water pump draws seawater to absorb heat from the

engine’s heat exchanger, preventing overheating.

2. The compression ratio in a diesel engine is typically:

A. 5:1 1 / 4

B. 8:1

• 14:1 to 22:1
• 25:1 to 30:1

Rationale: Diesel engines rely on high compression ratios between 14:1 and

22:1 to ignite fuel through heat of compression.

• What is the purpose of the fuel return line in a diesel system?
• To feed fuel injectors directly
• To vent air from the fuel tank
• To return excess fuel back to the tank
• To supply the lift pump

Rationale: Unused fuel and heat are returned to the tank to regulate

temperature and maintain pressure balance.

4. A leaking injector nozzle most likely causes:

• Hard starting
• White smoke
• High oil pressure
• Overheating

Rationale: Leaking injectors allow unburned fuel into the exhaust, producing

white smoke.

• / 4

• Which ABYC standard applies to diesel fuel systems?

A. ABYC A-4

B. ABYC H-33

C. ABYC E-11

D. ABYC P-1

Rationale: ABYC H-33 covers design, installation, and testing requirements

for diesel fuel systems.

• What does a high exhaust gas temperature indicate?
• Excess lubrication
• Overfueling or restricted air intake
• Low compression
• Proper combustion

Rationale: High EGT usually results from excess fuel or inadequate air

causing incomplete combustion.

7. The function of the governor in a diesel engine is to:

• Control oil pressure
• Regulate engine speed by adjusting fuel delivery
• Control water flow
• Adjust compression ratio

Rationale: Governors automatically adjust the fuel rack to maintain desired

engine speed under load changes. 3 / 4

8. The term “bleeding the fuel system” means:

• Adding fuel additive
• Cleaning the injectors
• Removing air from the fuel lines
• Replacing the lift pump

Rationale: Air in the fuel system prevents injection; bleeding ensures

consistent fuel delivery.

9. Diesel fuel contamination by water can cause:

• High RPM
• Blue smoke
• Microbial growth and injector damage
• Lower compression

Rationale: Water promotes microbial growth and corrosion, damaging

filters and injectors.

• What is the most accurate method to check bearing clearance?
• Feeler gauge
• Ruler
• Plastigage
• Micrometer
• / 4