by Scarlett GauthierUpdated March 16, 2018
Water is essential for a boat’s outboard motor to function properly. Water flows through the engine’s cooling system using a water pump and circulates through the power head before exiting through the exhaust system. Without it, an outboard motor will overheat. Overheating of outboards can be caused by various parts of the water cooling system. Careful regular maintenance of 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. Verify the lack of raw water flow by observing the temperature of the outlet side of the system. A water flow problem is indicated if the water temperature is excessively high. The lack of fresh water can be noticed if there is a large rise in temperature between the inlet and outlet of the heat exchanger. Water pump failure can also be caused by a faulty heat exchanger, although this is more difficult to verify. Generally, if it is not a raw or fresh water flow problem, the heat exchanger will be the cause.
The impeller, the device used to push the water down to the bottom of the power head, is a common cause of overheating. A corroded impeller will no longer allow the water pump to function properly. You can also occasionally drop a few shovels, in which case it is recommended to trace the flow of water to find all the pieces. If the intake pump is starving for a few moments, the rubber impeller may be damaged. However, it is also common for impellers to fail due to their old age. To avoid damage, the water impeller should be replaced every two years regardless of use. It is more common for an impeller to fail from rotting from lack of use than from wear and tear from overuse.
Manifolds and risers
Exhaust manifolds and risers can clog and restrict the flow of water through the engine. Although this is more common in salt water, it is still a possibility and cause of overheating. Rust and corrosion build up and clog water lines. You can check this by touching the risers when the engine is running. If it is noticeably hot to the touch, it is a sign that it is clogged. Before touching it, make sure it is cool enough not to burn your hand.
Check essential fluids like engine oil regularly. Often times, an outboard motor overheats because the oil has not been changed often enough to allow the pistons to receive the proper amount of lubrication.
Although the thermostat is not typically the cause of outboard motors overheating, it can be damaged by intense heat. Because it plays a crucial role in keeping the engine at a satisfactory temperature, allowing the engine to run, a damaged or faulty thermostat can cause overheating. Without a functional thermostat, an outboard motor will suffer poor performance that can lead to permanent internal damage.