How often should I replace my outboard water pump impeller?

Water pump impeller: when to replace it · Symptom: Service is due, the tell-tale stream is weak, or the engine ran hot or dry.

Quick answer: Inspect the water pump impeller every year and replace it every 1 to 3 seasons, or right away if the motor overheated or ran dry. Suzuki's guideline is inspect at 200 hours or 1 year and replace at 300 hours or 3 years; Mercury lists 300 hours or 3 years on many models; many mechanics replace it every season or two in salt, sandy or muddy water. Your owner's manual interval wins.

Signs it's due (most to least common)

Weak, sputtering, or no tell-tale stream even after clearing the outlet and intake screens
Temperature creeping up, overheat alarm, or engine dropping into a low-RPM warning mode
The motor was started or cranked out of water, or ran with a blocked intake
Long storage: vanes take a permanent bend ("set") and the rubber goes brittle
Regular running in sand, silt, mud, or salt water
Burning-rubber smell, or bits of black rubber coming out with the cooling water

Safe owner checks & fixes

Never start or crank the motor without water supplied (in the water, a test tank, or flush muffs as your manual allows). A few seconds dry can ruin an impeller.
Rule out the easy stuff first: clear the tell-tale outlet with a wire or zip-tie and clear the lower-unit intake screens. A plugged outlet looks like a bad pump.
Check your owner's manual interval and your hour meter, and write down when the pump was last serviced. If you don't know, treat it as due.
After any overheat or dry run, replace the impeller. Most shops replace the whole pump kit (impeller, gaskets, wear plate, and the housing or cup if it's scored).
DIY only with the factory service manual: the gearcase (lower unit) has to come off. Follow its steps for the shift shaft, impeller rotation direction, and sealants, and lube the vanes on install.
After the job, run it with water supplied and confirm a strong tell-tale stream and no leaks before you head out.
This isn't an on-the-water fix on an outboard. If the stream quits while you're out, shut down and get a tow instead of running it hot.

Parts / tools

Model-specific water pump kit (impeller, gaskets, wear plate; housing or cup if scored), factory service manual, sockets, the grease and sealant your manual specifies, flush muffs or test tank.

Call a marine mechanic when

You don't have the service manual or tools to drop the gearcase; the engine overheated badly or ran hot for more than a few seconds; impeller pieces are missing (they can lodge in the cooling passages); the stream is still weak with a new pump. If you're on the water and unsafe, call TowBoatUS/Sea Tow or the Coast Guard (VHF 16).

Sources

Values not stated here are model-specific; check your owner's manual.