What the impeller does — and why it fails
Your outboard is cooled by raw water, and the only thing moving that water is a small rubber impeller spinning inside the water pump housing on the lower unit. Its rubber vanes flex against the housing wall thousands of times a minute, pulling water up to the powerhead.
Rubber wears out. Vanes take a set from sitting, crack from age and heat, and get chewed up by sand. When the impeller stops moving enough water, your engine overheats — and an overheated outboard can mean a warped head or a ruined powerhead. One of the cheapest parts on the engine protects the most expensive one.
Signs your water pump is going
One related symptom worth knowing: milky or gray lower unit oil is not an impeller problem — it means water is getting into your gear oil past a seal, often from fishing line wrapped at the prop shaft. It's a separate repair, but we check for it on every water pump job since we're already working on the lower unit.
Why Florida boats eat impellers
Three reasons. Sand — running shallow, launching at ramps, and idling over bars pulls abrasive sediment through the pump, and it sands down the vanes from the inside. Salt — it corrodes the housing and dries the rubber. And dry starts — even a few seconds running an engine without water burns vanes almost instantly. A Florida boat that runs the flats and sits in salt water needs its pump serviced on the early side of the schedule.
When to replace it
The standard interval is every 300 hours or 3 years, whichever comes first — and we inspect it at every 100-hour service. Replace it sooner if you see the warning signs, if you've had a dry start, or if you run shallow, sandy water regularly.
A full water pump service means the impeller plus the wear parts around it: housing or liner, gaskets, O-rings, and keys. Dropping a new impeller into a scored housing is half a job.
What a mobile replacement looks like
We come to your dock, lift, marina slip, or driveway. The lower unit comes off, the old pump comes apart — and you can see exactly what your impeller looked like — new pump kit goes in, lower unit goes back on, and we run the engine to verify a strong tell-tale and normal operating temperature before we leave. Most single-engine jobs are done in one visit, and your boat never moves.
Most jobs run $240 – $500 depending on your engine model and whether the housing needs replacement along with the impeller. Use the instant estimator → or call (561) 602-1177 — tell us the make, model, and year and we'll give you a real number.
Tell-tale looking weak?
Don't wait for the alarm. Tell us your engine and we'll quote it today.
Get a quoteFAQ
How much does impeller replacement cost?
Typically $240–$500 depending on engine make and model, and on whether the pump housing needs replacement along with the impeller kit. Multi-engine boats are quoted per setup. Tell us your engine make, model, and year and we'll quote it before we schedule — call (561) 602-1177.
What are the signs of a bad water pump impeller?
A weak or irregular tell-tale stream, engine temperature climbing at idle or trolling speed, and the overheat alarm. If the alarm sounds, shut the engine down — don't try to run it home. Continued overheating is how a pump job turns into a powerhead job.
Why is my lower unit oil milky?
Milky gear oil means water is getting past a seal into the lower unit — commonly from fishing line cutting the prop shaft seal. It's a gear-oil and seal issue, not an impeller issue, but it needs attention fast: water in the gearcase destroys gears and bearings. We inspect for it on every water pump job.
How often should the impeller be replaced?
Every 300 hours or 3 years, whichever comes first — that's the interval Yamaha, Mercury, and Suzuki all land near. Florida boats that run sandy shallows or sit in salt water year-round often need it sooner. We inspect the pump at every 100-hour service so you're never guessing.
Can you replace the impeller without hauling the boat out?
In most cases, yes. On a lift or trailer it's straightforward. For boats in the water, it depends on access to the lower unit — tilted up at a dock is often workable. Tell us where the boat sits and we'll confirm before we schedule.