MST Marine Engine Diagnostics & Repair 5

Question 1

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?

Accepted Answer

Heat exchanger core for lodged vane pieces

Answer

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.

Question 2

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?

Accepted Answer

Too low a spark plug gap or fouled plugs

Answer

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.

Question 3

When reinstalling a marine engine cylinder head, what is the correct sequence for torquing head bolts?

Accepted Answer

Inside to outside in a spiral pattern from the center

Answer

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.

Question 4

An EFI marine engine idle mixture cannot be adjusted with the idle air control valve at its limits. What should the technician check?

Accepted Answer

Intake manifold vacuum leaks or air leaks after the mass airflow sensor

Answer

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.

Question 5

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?

Accepted Answer

Manually prime the low-pressure circuit with the hand primer, then crack open high-pressure line fittings at the injectors while cranking

Answer

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.

(MST) Marine Service Technician Practice Test

(MST) Marine Service Technician Practice Test

MST Marine Engine Diagnostics & Repair 5