An outboard’s water pump pushes cooling water from the lower unit up through the powerhead, and its heart is a flexible rubber impeller. When the impeller wears, cracks or loses vanes, the telltale stream weakens, the motor runs hot and the overheat alarm may sound. Most makers call for replacing the impeller at regular intervals, and running the motor without water ruins it in seconds.
How an outboard water pump works
The impeller sits in a housing on top of the lower unit, keyed to the driveshaft. As it spins, its rubber vanes flex against an offset housing and push water up the water tube to the engine. Water leaves through the exhaust and the small telltale outlet, which lets you see that the pump is working.
Signs of a failing water pump or impeller
- Weak or no telltale stream: check the telltale hole for blockage first; a piece of wire often clears it.
- Overheat alarm or warning light, or the motor dropping into a reduced-power mode.
- Steam or a hot smell from the powerhead.
- Pieces of rubber vane coming out of the cooling passages.
Outboard overheating: common causes
- A worn or damaged impeller, or a cracked pump housing
- Blocked water intake screens from weeds, mud or a plastic bag
- A stuck thermostat or clogged cooling passages, especially after saltwater use without flushing
- Running in very shallow, sandy water, which wears the impeller quickly
- Running on flush muffs without the water turned fully on
Shut the motor down if it overheats. Running it hot can damage pistons and head gaskets, the same as in a car engine; see engine overheating.
When to replace the impeller
Replacement intervals vary by maker and use, so follow your owner’s manual. Many owners replace the impeller every few seasons as preventive maintenance, and sooner after running in sandy or silty water or after any overheating event. Impellers also take a set when a motor sits for long periods, which is another reason to replace them on schedule.
How the impeller is replaced
- Disconnect the battery or remove the kill switch clip, and shift to neutral.
- Remove the lower unit (gearcase) following the service manual, noting the shift linkage.
- Remove the pump housing, then the old impeller and its drive key.
- Inspect the housing and wear plate for grooves; replace them if worn, as a kit usually allows.
- Fit the new impeller with the vanes turned in the direction of rotation, lightly lubricated as the instructions say, and reassemble with new gaskets.
- Reinstall the lower unit and check for a strong telltale stream on the first run.
With the lower unit off, it is a convenient time to change the gear oil; see how to change lower unit oil.
Preventing water pump problems
- Never run the motor out of water, even briefly.
- Flush the motor with fresh water after saltwater or muddy use, as part of routine outboard motor maintenance.
- Check the telltale every time you start the motor.
- Include the impeller in your winter service; see how to winterize a boat and outboard motor.
Replacing an older motor instead? Browse new Suzuki, Tohatsu and Mercury four-strokes on our outboard motors by horsepower page.
More guides: outboard overheating · outboard loses power at full throttle · milky lower unit oil · how to flush an outboard · outboard not peeing · zinc vs aluminum anodes · outboard thermostat.
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