MST Marine Engine Diagnostics & Repair 2

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?

Accepted Answer

Leaking valve stem seals

Answer

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?

Accepted Answer

Worn or broken piston rings

Answer

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?

Accepted Answer

Retard the injection timing

Answer

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?

Accepted Answer

Dial indicator with magnetic base

Answer

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?

Accepted Answer

Partially clogged cooling water passages

Answer

Partially clogged cooling passages restrict flow sufficiently at high demand (high RPM) but allow adequate flow at idle, causing overheating only under load.

(MST) Marine Service Technician Practice Test

(MST) Marine Service Technician Practice Test

MST Marine Engine Diagnostics & Repair 2