Boat Diesel – The engine is overheating

The piston and connecting rod are usually the first major components to become damaged if the engine overheats. Worn cylinders and broken rings can cause low compression, reluctance to start and excessive oil consumption. Big end wear can be caused by low oil pressure. 1 Compression rings. 2 Oil control rings. 3 Piston. 4 Gudgeon pin. 5 Circlips. 6 Piston skirt. 7 Small end bearing. 8 Connecting rod. 9 Bolt. 10 Big end bearing.

An overheating exhaust can melt the water trap, burn the hoses and cause a fire. A waterlock system at its simplest
comprises: 1 Cooling water pipe looped above waterline. 2 Small bore pipe for bleed overboard, or a vacuum relief
valve can be fitted at 3. 4 Water injection bend. 5 Waterlock/silencer chamber. 6 Outlet pipe. 7 Exhaust shut-off valve.

The common symptoms of an overheated engine are:
1. Engine noise changes. It may sound laboured.
2. The temperature gauge shows an increase over normal.
3. Smoke. This can be caused by a burning exhaust and/or, if the exhaust is water cooled, the plastic water trap melting.
4. No water coming out of the exhaust or water outlet.
5. The engine stops, or seizes completely.

Cooling water flow to the engine can be interrupted for a number of reasons:
1.An obstruction such as plastic bag blocking the water intake. This can be difficult to pinpoint, but when the engine is stopped the obstruction may float free. A water inlet filter with a clear top will enable the incoming water supply to be monitored.
2.The intake valve may be closed or obstructed. Debris such as barnacles and weed can gather in the valve. Valves need to be checked and cleaned when laid up. Once again, an inlet filter with a clear top will help.

Easy access to all parts of the engine will make routine maintenance easier and emergencies less traumatic. This engine installation aboard the author’s boat is some years old.

3.Accumulation of salt deposits and debris in the engine waterways. These can build up and stop the water circulating effectively. This is always a problem in raw-water-cooled engines. Good laying up procedures (see Chapter 13) can extend the period between major cooling system overhauls. The effectiveness of a heat exchanger in indirect systems can also be reduced by salt and debris.

If the engine is overheating, check that the cooling water intake valve is open (handle in line with pipe).

The best tool to remove an impeller is a slipjoint wrench. Impellers come in many sizes, some with shaft locking spindles that must be unscrewed before the impeller can be freed. On reassembly make sure the pump body faces are clean, fit the blades with the correct ‘set’, fit a new gasket and grease the impeller.

4.The pump impeller may be damaged or broken. Most pumps use a rubber vane-type impeller. These have a hard life and eventually the vanes may break off. Impellers are normally easy to change but you must always retrieve any rubber debris and broken vanes from the system to avoid blocking the waterways.

The impeller may also fail if the bond between the metal centre and the rubber fails. The impeller is one of the first casualties of an engine that is starved of water. Since it depends on water for lubrication, if the pump has run dry check the impeller. If it is damaged replace it, making sure to give the vanes the correct initial set, according to the direction of rotation. A dab of light grease or petroleum jelly will help.

5.Failure of the thermostat. If the thermostat fails closed it will prevent cooling water circulating properly around the engine, and overheating can result. If you suspect the thermostat, remove it temporarily. The engine will run cooler, and therefore less efficiently, but should not suffer any damage if run for a short period.

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