An outboard overheats when not enough cooling water is flowing through the powerhead. The most common causes are a worn water pump impeller, a blocked water intake, a stuck thermostat, a clogged telltale passage and running in shallow, sandy or weedy water. Stop the engine when the overheat alarm sounds, because continuing can damage pistons and cylinders.
Outboard overheating symptoms
- An overheat horn, warning light or the motor limiting its speed
- A weak or missing telltale stream (the small water stream from the side or back of the motor)
- Steam or a hot smell from the cowling
- Loss of power and rough running
Common causes
| Cause | Clues |
|---|---|
| Worn water pump impeller | Weak telltale, worse at idle; see outboard water pump impeller |
| Blocked water intake | Weeds, plastic bags or mud on the lower unit grates |
| Stuck thermostat | Overheats at idle or low speed even with good flow |
| Clogged telltale | No stream, but the motor is not hot; clear it with a thin wire |
| Salt or scale buildup | Gradual overheating in salt water without regular flushing |
Overheating but pumping water
If the telltale stream looks fine but the motor still overheats, suspect the thermostat, scale or salt buildup in the cooling passages, or a failing temperature sensor. A clogged telltale can also mislead you the other way, showing no stream while water still flows.
Overheating at idle vs at high rpm
Overheating at idle usually points to a worn impeller or stuck thermostat, because the pump moves the least water at low speed. Overheating at high rpm can mean a partly blocked intake, a damaged impeller that collapses under load, or the motor mounted too high so it draws in air.
Overheating on muffs
Flush muffs can slip or supply too little water, especially at higher rpm. Keep the motor at idle on muffs, make sure the water supply is strong and the cups seal over the intakes, and never run the motor without water.
What to do
- Throttle back and stop the engine when the alarm sounds.
- Tilt the motor and clear the intake grates.
- Check the telltale and clear it if blocked.
- Restart and watch for a strong stream; if it is still weak, replace the impeller before using the motor hard.
Prevent overheating
Replace the impeller on the schedule in your manual, flush after salt water use and inspect the intake before each trip; see outboard motor maintenance. If an older motor has been badly overheated and lost compression, compare a replacement on our outboard motors page.
More guides: hydrolocked engine · outboard loses power at full throttle · how to flush an outboard · outboard not peeing · outboard thermostat.
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