MST Marine Engine Diagnostics & Repair 2 — Questions and Answers
Question 1: A 4-stroke inboard engine exhibits excessive oil consumption with blue exhaust smoke at startup that clears after warm-up. What is the most likely cause?
- Worn piston rings
- Leaking valve stem seals (Correct answer)
- Overfilled crankcase
- Clogged PCV valve
Correct answer: Leaking valve stem seals
Blue smoke at cold startup that clears when warm is a classic sign of worn or leaking valve stem seals allowing oil to drip into the cylinders when the engine is off.
Question 2: When performing a cylinder leakage test on a marine engine, a reading of 25% leakage is found. Air is heard escaping into the crankcase. What does this indicate?
- Blown head gasket
- Worn or broken piston rings (Correct answer)
- Leaking intake valve
- Cracked cylinder head
Correct answer: Worn or broken piston rings
Air escaping into the crankcase during a leakdown test indicates worn or broken piston rings allowing combustion gases to bypass the piston.
Question 3: A marine diesel engine produces black smoke under load. The fuel injection timing was recently advanced. What is the most appropriate corrective action?
- Replace the injectors
- Retard the injection timing (Correct answer)
- Increase the fuel pressure
- Clean the air filter
Correct answer: Retard the injection timing
Black smoke under load after advancing timing indicates over-fueling relative to air supply; retarding injection timing reduces combustion temperature and incomplete combustion.
Question 4: Which tool is used to measure crankshaft end play on a marine engine?
- Micrometer
- Dial indicator with magnetic base (Correct answer)
- Feeler gauge
- Plastigage
Correct answer: Dial indicator with magnetic base
A dial indicator mounted on a magnetic base measures crankshaft end play by prying the crank axially and reading the total movement on the gauge.
Question 5: A stern drive engine overheats only at high RPM and normal operating temperature is maintained at idle. What is the most likely cause?
- Stuck open thermostat
- Partially clogged cooling water passages (Correct answer)
- Faulty temperature gauge sender
- Low engine oil level
Correct answer: Partially clogged cooling water passages
Partially clogged cooling passages restrict flow sufficiently at high demand (high RPM) but allow adequate flow at idle, causing overheating only under load.
Question 6: What is the purpose of a sacrificial zinc anode on a marine outdrive unit?
- To lubricate the propeller shaft
- To prevent galvanic corrosion of drive components (Correct answer)
- To reduce cavitation around the propeller
- To act as a grounding point for electrical accessories
Correct answer: To prevent galvanic corrosion of drive components
Sacrificial zinc anodes corrode preferentially in the galvanic series, protecting more noble metals like aluminum and steel in the outdrive from electrochemical corrosion.
Question 7: A two-stroke outboard engine runs well at idle and wide-open throttle but stumbles in the mid-range. Which carburetor circuit is most likely at fault?
- Main jet circuit
- Needle and needle jet circuit (Correct answer)
- Pilot/slow jet circuit
- Choke circuit
Correct answer: Needle and needle jet circuit
The needle and needle jet circuit controls fuel delivery in the mid-throttle range (approximately 1/4 to 3/4 throttle), so mid-range stumble points to this circuit.
MST Marine Engine Diagnostics & Repair 2
A 4-stroke inboard engine exhibits excessive oil consumption with blue exhaust smoke at startup that clears after warm-up.
What is the most likely cause?