What Causes an Inboard/Outboard Motor to Overheat?


Overheating problems in an inboard-outboard motor, also called a "stern-drive," happen as a result of component failure somewhere in the cooling system. One part might be responsible, or a number of parts can contribute to the overheating problem. Even mild overheating can cause serious damage to expensive engine components. Upon any sign of overheating, a boat owner should investigate the root cause of any abnormally high temperatures.

Spark Plugs

  • Installing a spark plug that has the incorrect thread length into a marine engine head can cause a run-away pre-igntion problem. Spark plug threads that extend into the combustion chamber can accumulate heavy carbon deposits, which continue to burn past the combustion cycle. As a result, the plug electrode overheats and causes higher than normal combustion temperature.

Carbonized Combustion Chambers

  • The inboard-outboard cylinder head can accumulate large carbon deposits in the combustion chamber head, valve surfaces, around the exhaust valve stem and in the exhaust ports. The heavy accumulations form a thick barrier that resists normal heat dispensation, reducing the combustion chamber volume and absorbing and holding heat after ignition. De-carbonizing sprays use strong solvents, which when injected into the fuel intake system break up and dislodge the carbon.

Intake Restriction

  • Overheating of the inboard-outboard motor at the raw water intake ports on the lower unit can block off the cooling water to the engine. When the small intake ports on the lower unit become clogged with twigs, mud, seaweed or plastic, they can starve the engine for cooling water. Boat owners should inspect the intake ports and free the ports, or intake screens, of all debris. The trim setting (motor angle) should not be tilted up excessively high where the ports break the water's surface and suck air.

Radiator and Thermostats

  • Some inboard-outboard marine engines contain a closed loop cooling system that consists of a radiator, which uses anti-freeze (coolant) for the purpose of cooling the engine passages and wet exhaust manifold. Overheating problems manifest when the radiator cores become clogged with rust and residual oil and sludge. A defective thermostat stuck in the closed position will completely block the cooling fluid from circulating in the engine, leading to rapid overheating.

Water Pump Impellers

  • Rubber water pump impellers are used on all inboard-outboard marine engines. The impellers have fan-like blades that rotate rapidly to draw intake water through a raw water inlet and pump it through the various engines passages and exhaust (wet) manifold jackets. If the rubber blades turn brittle with age they no longer hold their seal within the water pump housing, causing a reduced flow of cooling water. Broken impeller blades can also reduce the flow and plug a water jacket on the discharge side of the water pump.

Gearcase Oil

  • Low and-or contaminated lower gear case oil will cause an overheating problem. If the gear case oil level measures below specifications, it can cause inadequate lubrication to the transmission drive gears, which will raise the temperature of the lower unit and transmit the heat through the engine case. Water that has contaminated the gear case oil because of ruptured seals or gaskets will breakdown the viscosity of the oil, keeping it from lubricating the parts. Improper lubrication produces friction, which produces excess heat.

Ambient Air Temperature and Full Throttle

  • Outside ambient air temperatures that exceed triple digits will cause an elevation of overall engine heating, especially under dry humidity conditions. Combined with running the engine at full throttle for prolonged periods, the engine suffers more load on all the major engine components, like the crankshaft bearings, cam, valve guides, pistons, rings and rod bearings. Constant full-throttle operation on very hot days, should be avoided, especially if towing skiers or carrying heavy passenger loads.

Head Gaskets

  • A blown head gasket in an inboard-outboard engine will cause water to enter the combustion chamber and be discharged out of the exhaust. The loss of cooling water that would normally circulate throughout the engine becomes reduced, overheating the affected cylinders. Blown head gaskets require major engine repair and expense.


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