Marine Engine Diagnostics & Repair Flashcards
7 cards from real MST practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Marine Engine Diagnostics & Repair flashcards as text
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?
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.
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?
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.
A marine diesel engine produces black smoke under load. The fuel injection timing was recently advanced. What is the most appropriate corrective action?
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.
Which tool is used to measure crankshaft end play on a marine engine?
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.
A stern drive engine overheats only at high RPM and normal operating temperature is maintained at idle. What is the most likely cause?
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.
What is the purpose of a sacrificial zinc anode on a marine outdrive unit?
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.
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?
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.