Water is critical for a boat's outboard motor to function properly. Water flows through the motor’s cooling system using a water pump and circulates throughout the powerhead before exiting through the exhaust system. Without it, an outboard motor will overheat. Overheating in outboard motors can be caused by any of various parts of the water cooling system. Vigilant regular maintenance on your outboard is your best insurance against a cooling system overheating problem.
Water Pump Failure
The most common cause of outboard motor overheating is water pump failure. Lack of raw water flow and lack of fresh water can cause water pump failure. Check for lack of raw water flow by noting the temperature of the outlet side of the system. A water flow problem is indicated if the water temperature is excessively high. Lack of fresh water can be noted if there is a large increase in temperature between the inlet and and outlet of the heat exchanger. Water pump failure can also be caused by a defective heat exchanger, though this is harder to check. Generally, if it is not a problem with lack of raw water flow or fresh water, the heat exchanger will be the cause.
The impeller, the device used to pressure the water into the bottom of the powerhead, is a common cause of overheating. A corroded impeller will no longer allow the water pump to function properly. It can also occasionally shed some blades, in which case tracing the water flow is recommended to find all the pieces. If the intake pump starves for water for a few moments, the rubber impeller may be damages. However, it is also common that impellers fail due to their old age. To prevent damage, the water impeller should be replaced every two years regardless of usage. It is more common for an impeller to go bad from rot due to nonuse rather than from wearing out from excessive usage.
Manifolds and Risers
Exhaust manifolds and risers can clog and restrict the flow of water through the engine. Although this is most common in salt water, it is still a possibility and cause of overheating. Rust and corrosion accumulate and clog the water passages. You can check this by touching the risers when the engine is running. If it is noticeably warm to the touch, this is a sign it is clogged. Before touching it, make sure it is cool enough so not to burn the hand.
Check on essential fluids such as the engine oil regularly. Often an outboard motor will overheat because the oil has not been changed often enough to allow the pistons to receive a proper amount of lubrication.
Though the thermostat is not often the cause of overheating in outboard motors, it can be damaged due to high heat. Because it plays a crucial role in keeping the engine at a satisfactory temperature, allowing the engine to function, a damaged or otherwise failing thermostat can cause overheating. Without a functional thermostat, an outboard motor will suffer from poor performance which can further lead to permanent internal damage.