The water pump is a small pump on the side of a liquid-cooled engine, driven by a shaft from inside the engine. It has a plastic or metal impeller in a housing, a mechanical seal on the shaft and usually a small drain hole (weep hole) under the housing.
What it does
The impeller spins and pushes coolant through the engine and radiator. The seal keeps coolant out of the engine oil on one side and in the pump on the other. If the seal wears, coolant drips from the weep hole or gets into the oil. Hard deposits from tap water or wrong coolant wear the seal faster; a broken impeller stops flow.
Signs it is worn or failing
- Coolant dripping from the weep hole: Coolant is leaking.
- Milky oil from a failed inner seal.
- Liquid-cooled engine overheats with a full system and a working fan.
- Whining noise from the engine from a worn shaft bearing.
When to replace it
When the weep hole drips, the seal leaks into the oil, the shaft has play, or the impeller is cracked. Many pumps are sold as a complete unit; some have a seal kit. Follow your bike's manual.
What to match when buying
- Match make, model and year, or the engine code. Impeller size and shaft drive differ.
- Buy the housing gasket and O-rings with it.
- Use the right coolant afterwards; Honda warns that silicate coolant can wear water pump seals.
Fitting notes
- Workshop job. Drain coolant cold, and change the oil too if coolant got into it.
- Do not touch the seal faces; fit dry unless the maker says otherwise.
- Refill and bleed: Replace the coolant. Check for leaks: Find a coolant leak.