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 marine engine's raw water pump impeller is found to have several broken vanes. Which additional component must always be inspected and likely replaced at the same time?
Answer: Heat exchanger core for lodged vane pieces
Broken impeller vanes travel downstream and commonly lodge in the heat exchanger tubes, restricting flow; failure to remove them causes the new impeller to overheat quickly.
A technician uses an oscilloscope to analyze a marine engine's primary ignition waveform and notices the burn time is shorter than specification. What does this indicate?
Answer: Too low a spark plug gap or fouled plugs
Short burn time on the primary waveform indicates the spark is not sustaining long enough, typically caused by a spark plug gap that is too small or plugs that are fouled and offer less resistance.
When reinstalling a marine engine cylinder head, what is the correct sequence for torquing head bolts?
Answer: Inside to outside in a spiral pattern from the center
Head bolts must be torqued from the center outward in a spiral pattern to evenly distribute clamping force across the gasket and prevent warping the head.
An EFI marine engine idle mixture cannot be adjusted with the idle air control valve at its limits. What should the technician check?
Answer: Intake manifold vacuum leaks or air leaks after the mass airflow sensor
When the IAC is at its limit and cannot correct idle mixture, unmetered air entering after the MAF sensor is a common cause, as the ECU cannot account for this additional air in its fuel calculations.
A marine diesel's fuel system has been serviced and the engine will not start. Air is suspected in the high-pressure side of the fuel system. What is the correct bleeding procedure?
Answer: Manually prime the low-pressure circuit with the hand primer, then crack open high-pressure line fittings at the injectors while cranking
Diesel fuel systems must be bled by priming the low-pressure side first with the hand pump, then loosening injector line fittings to allow trapped air to escape while cranking.
What does a low vacuum reading (8–12 in. Hg) that is steady at idle indicate during an engine vacuum test on a marine engine?
Answer: Retarded ignition timing or late valve timing
A steady but abnormally low vacuum reading at idle typically indicates retarded ignition timing or cam timing that is late, reducing the engine's pumping efficiency.
A marine technician is replacing the crankshaft rear main seal on an inboard engine. After installation, the new seal leaks immediately. What is the most likely cause?
Answer: Crankshaft journal surface is grooved or damaged at the seal lip contact area
A grooved or worn crankshaft journal surface creates a path for oil to travel past the seal lip, causing immediate leakage even with a correctly installed new seal.