MTC Marine Technician Fuel & Cooling Systems 3 — Questions and Answers
Question 1: A marine technician notices white powdery deposits around coolant hose clamps on a raw-water-cooled engine. This MOST likely indicates:
- Normal oxidation of the clamp material
- Coolant leakage from a pressurized joint
- Raw water seeping past a loose clamp connection (Correct answer)
- Exhaust gas intrusion into the cooling circuit
Correct answer: Raw water seeping past a loose clamp connection
White mineral deposits (scale) around hose clamps signal raw water seepage; raw water evaporates and leaves mineral residue, unlike antifreeze which leaves a sticky colored stain.
Question 2: What is the purpose of a fuel/water separator in a marine diesel fuel system?
- To emulsify water so it burns safely with the fuel
- To remove entrained water before it reaches injection components, preventing corrosion and poor combustion (Correct answer)
- To pressurize the fuel supply above lift-pump pressure
- To filter particulates larger than 2 microns only
Correct answer: To remove entrained water before it reaches injection components, preventing corrosion and poor combustion
Water in diesel fuel damages injection pumps and injectors; a fuel/water separator coalesces and collects water in a bowl that must be drained regularly.
Question 3: On a stern-drive engine, raw water enters the cooling system through the drive unit lower unit and exits via the exhaust. If the technician sees no water exiting the exhaust at idle, the FIRST component to inspect is:
- The thermostat
- The raw-water impeller (Correct answer)
- The heat exchanger
- The expansion tank cap
Correct answer: The raw-water impeller
No exhaust water at idle almost always points to a failed raw-water impeller, as this pump is the first component that must move water before it can reach the exhaust outlet.
Question 4: Which type of fuel tank vent system is required by ABYC standards to prevent siphoning of fuel overboard?
- A straight open vent tube directed downward
- A looped vent line with its highest point above the fuel tank fill neck level (Correct answer)
- A pressurized sealed tank system
- A one-way check valve vented directly to the bilge
Correct answer: A looped vent line with its highest point above the fuel tank fill neck level
ABYC E-11 requires the fuel vent line to loop above the fuel tank fill level so fuel cannot siphon out through the vent if the vessel heels or the vent is submerged.
Question 5: A technician measures intake manifold vacuum at idle and finds it cycling between 15 and 0 in. Hg. What fuel system fault could cause this reading?
- Lean air-fuel mixture due to a vacuum leak
- A sticking injector causing intermittent fuel enrichment
- A collapsing fuel supply line creating intermittent fuel starvation (Correct answer)
- High fuel rail pressure from a faulty regulator
Correct answer: A collapsing fuel supply line creating intermittent fuel starvation
A collapsing fuel line collapses under pump suction, momentarily cutting fuel flow and causing the engine to stumble; the line re-opens and flow resumes, producing a cycling vacuum gauge.
Question 6: In a marine closed-cooling system, what is the purpose of the recovery/overflow bottle?
- To store reserve raw water for emergency cooling
- To capture coolant expelled when hot and return it to the system as it cools, maintaining correct coolant level (Correct answer)
- To pressurize the system above atmospheric pressure
- To mix fresh coolant concentrate with water before use
Correct answer: To capture coolant expelled when hot and return it to the system as it cools, maintaining correct coolant level
As the engine heats up and coolant expands past the cap's pressure rating, it overflows into the recovery bottle; when the engine cools, it is drawn back in, preventing air ingestion.
Question 7: What condition can cause a marine engine's cooling water tell-tale (indicator stream) to produce a weak, intermittent stream even when the impeller is new?
- Incorrect thermostat temperature rating
- A partially blocked tell-tale orifice or hose kink in the indicator line (Correct answer)
- Excessive raw-water pump impeller vane count
- High ambient water temperature
Correct answer: A partially blocked tell-tale orifice or hose kink in the indicator line
The tell-tale orifice is small and easily clogged by marine debris or scale, producing a weak or intermittent stream even when the impeller pumps full flow.
MTC Marine Technician Fuel & Cooling Systems 3
A marine technician notices white powdery deposits around coolant hose clamps on a raw-water-cooled engine.
This MOST likely indicates: