Marine Technician Fuel & Cooling Systems Flashcards

A marine diesel engine surges at cruise RPM but runs smoothly at idle and full throttle. The MOST likely cause is:

Answer: Hunting by the mechanical governor at mid-speed settings

Mechanical governor hunting — oscillating between richer and leaner fuel delivery — is a classic cause of mid-speed surge; the governor is stable at the extremes of its range but unstable in the mid-range.

Which material is MOST commonly used for marine raw-water pump impellers due to its flexibility and resistance to seawater?

Answer: Neoprene (polychloroprene)

Neoprene impellers are the marine industry standard because neoprene is flexible enough to create a pumping action against the eccentric housing and resists degradation from seawater and mild fuel contamination.

When bleeding air from a marine diesel fuel system after filter service, which sequence is correct?

Answer: Bleed the filter housing, then the injection pump inlet, then the injectors

Air must be removed progressively from the lowest-pressure (filter) to highest-pressure (injection pump inlet) to injector bleed points, working downstream to fully purge the circuit.

A marine outboard engine overheats at high speed but cools normally at idle. With a new impeller installed, the NEXT component to suspect is:

Answer: Scale or blockage in the water passages of the powerhead

At high speed the engine demands more cooling flow; scale-blocked internal water passages restrict flow sufficiently at high load but allow adequate flow at low demand/idle.

What is the recommended procedure when storing a marine outboard for winter in a freezing climate regarding the cooling system?

Answer: Flush with fresh water, then run the engine on flushing muffs until all water is expelled or introduce antifreeze via the water intake

Proper winterization requires flushing with fresh water to remove salt and debris, then either running until dry or pumping antifreeze through the system to prevent freeze cracking.

In a marine carburetor, the function of the accelerator pump is to:

Answer: Inject a short burst of extra fuel when the throttle is rapidly opened to prevent stumbling

Rapid throttle opening momentarily leans the mixture as air velocity increases faster than fuel flow; the accelerator pump squirts extra fuel to fill this gap and prevent hesitation.

What does a rising coolant temperature combined with a dropping oil pressure reading on a marine inboard engine MOST likely indicate?

Answer: Severe overheating causing oil viscosity loss, or a blown head gasket allowing coolant into the oil

Rising temperature with falling oil pressure suggests oil is becoming dangerously thin from extreme heat or is contaminated with coolant, both requiring immediate engine shutdown.