An outboard thermostat controls how much cooling water flows through the powerhead so the engine runs at its designed temperature. One stuck closed causes overheating and the warning horn, often worst at idle and low speed. One stuck open makes the engine run cold, with rough idle, slow warm-up, poor fuel economy and, on two-strokes, fouled plugs. Most outboards have the thermostat under a small cover near the top of the cylinder head, and V engines often have one per bank.
Stuck closed symptoms
- Overheat alarm or warning light, especially at idle or trolling speed.
- Weak or very hot tell-tale stream.
- Engine power reduced by the overheat protection on newer engines.
Some engines have a pressure relief valve that opens at higher rpm, so a stuck thermostat may show mainly at low speed. Overheating has other causes too, a worn impeller above all; see outboard overheating and outboard water pump impellers.
Stuck open symptoms
- Temperature gauge reads low or the engine is slow to warm up.
- Rough or unstable idle and poor low-speed running.
- Higher fuel use, and fouled plugs on two-strokes; see outboard spark plugs.
How to test an outboard thermostat
- Remove the thermostat cover and the thermostat, noting which way it faces.
- Check it for salt, scale or debris holding it open.
- Suspend it in a pan of water with a thermometer and heat it slowly.
- It should start to open near the temperature stamped on it, or listed in the service manual, and close again as it cools.
Replace it if it does not open, opens at the wrong temperature or will not close fully. Fit a new gasket every time the cover comes off.
Running without a thermostat
Removing the thermostat does not cure overheating. The engine runs too cold, which causes carbon buildup, fuel dilution of the oil on four-strokes, poor idle and faster wear. Find the real cause of overheating instead.
Salt water and maintenance
Salt and scale can jam a thermostat, so many owners check or replace it at the same interval as the water pump impeller. Flushing after every saltwater trip helps; see how to flush an outboard. Some engines also have an anode in or near the thermostat housing; see zinc vs aluminum anodes.
Related
If the tell-tale is weak or missing, see outboard not peeing. For car engines, see thermostat stuck open or closed. Browse engines on outboard motors.
More guides: outboard ignition switch.
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