Outboard water pump impeller: signs it's worn, how often to change it and how surveyors write it up

An outboard is cooled by the water it sits in, pushed through the powerhead by a small rubber impeller in the lower unit. BoatUS notes that most boat owners have no idea when the pump on their outboard was last serviced, and some don't know there is one. When it fails, the engine overheats, and that can mean permanent damage.

How a surveyor checks it

The tell-tale. With the engine running, watch the small stream from the tell-tale outlet on the leg or cowl. It should be steady. Weak, intermittent or missing means the pump isn't moving enough water, or the tell-tale itself is blocked.

The temperature. Note any overheat warnings on the sea trial and ask about the engine's history.

The records. BoatUS says the manual should give the service interval, but often every other year is about right. An impeller with no record is due.

If the pump is opened. In BoatUS's walkthrough the vanes of the old impeller had taken a set, a bend, after several months in the pump, compared with the straight vanes of a new one. Look for missing or cracked vanes and a scored housing.

What decides the grade

Water flow. No stream, or an engine that runs hot, is an A: don't run it until the pump is serviced. A weak stream is a B. A good stream with no service record is a C.

What servicing involves

The gearbox oil is drained to drop the lower unit, so it's a good time to look at the oil for water.

Photos to take

The tell-tale stream with the engine running, the leg and cavitation plate, and any service sticker or record. With Axsail, say what the tell-tale is doing as you film it and it goes on the outboard finding.

Writing it up

Say how the tell-tale stream looked at idle and at higher revs, any temperature warnings, and when the impeller was last changed. With an A, say "do not run the outboard".