Boat engine temperature alarm: secure the boat and find the cooling fault

Stop before a fault becomes a breakdown

Reduce load, secure the boat and follow the engine's alarm instructions. If overheating is confirmed or there is no cooling water flow: stop the engine. Continuing to run it can cause serious damage. Don't restart just because the alarm has gone quiet; the cause may still be there. Arrange a tow or assistance if you can't safely restore cooling.

Never open a hot, pressurised cooling system. Keep hands and tools away from moving belts and the propeller. Start with the model-specific troubleshooting.

Which cooling system do you have?

An outboard uses seawater for cooling. Many inboards have a closed coolant system cooled by seawater through a heat exchanger; others are raw-water cooled directly. That means an inboard can have both a raw-water flow problem and a coolant or circulation problem.

The tell-tale stream on an outboard is an indication, not a full guarantee that the engine is being cooled. A blocked tell-tale can give a weak stream without the whole cooling system having stopped, and a normal stream does not rule out every fault. Always check the engine's alarms and manual.

Start with the waterway

1. With the engine off: check that the water intake isn't blocked by seaweed, plastic or silt.
2. On inboards: check that the seacock is open and the raw water strainer isn't clogged. Close the seacock before opening the strainer, following the installation instructions.
3. Check visible hoses, connections and leaks. A leak on the suction side can draw in air.
4. Inspect the impeller if there's still no flow. If it's broken, find the missing vane pieces.
5. Refit the strainer lid and reset the valves correctly before any test start.

Read the impeller guide and find the right part under spare parts. Never run a raw water pump dry.

When the raw water system seems to be working

Let the engine cool down completely and check the coolant level if it has a closed system. Don't top up with an unknown fluid on the off chance; use the correct specification. The thermostat, circulation pump, heat exchanger and exhaust components may need a workshop inspection. A faulty sensor is possible, but don't assume it until real overheating has been ruled out.

Note whether the temperature rises only under load or at idle as well. Write down the alarm message, engine rpm, time and what happened before the fault. This information helps the workshop without you having to do more risky test runs.

When to stop troubleshooting yourself

Stop if there is heavy leakage, steam, mechanical noises or a recurring alarm after a simple fix. Have a workshop assess the engine if it has run hot for a long time, impeller pieces are missing or you can't check the system safely. An acknowledged warning doesn't mean the engine is healthy.

Identify the engine and serial number in the engine catalog and contact a boatyard. Ask for the root cause to be diagnosed and for a check after the repair. Just replacing an impeller can leave a blockage further into the system.

Frequently asked questions