When to Replace Your Raw Water Impeller

The raw water impeller is the heart of your marine engine’s cooling system. Whether you’re running a MerCruiser inboard, a Crusader V8, a Volvo Penta, or an outboard of any brand, that little rubber wheel is what keeps your engine from overheating every time you turn the key. But here’s the catch: waiting until it “looks bad” to replace it is often too late.

What the Raw Water Impeller Actually Does

The impeller pulls in seawater (or lake water) from the outside of the boat and pushes it through the engine’s cooling passages. In raw-water cooled engines, this water goes through the block, heads, manifolds, and risers. In closed-cooled systems, it goes through a heat exchanger. Either way — no water, no cooling, no engine.

Why Visual Inspection Isn’t Enough

Rubber fatigue happens before visible failure
Even if the vanes look intact, they may be stiff, cracked at the base, or deformed. Over time, heat and dry startup conditions degrade the material — long before it “looks bad.”
Impeller blades take a permanent set
Impellers sit compressed in one direction. If left too long, especially over winter, they lose flexibility and can’t maintain flow — even if they’re not broken.
Failure is usually catastrophic and sudden
When an impeller fails, it often loses multiple vanes at once. These fragments can lodge in hoses, thermostats, oil coolers, or manifolds — causing hidden blockages that persist even after replacement.

How Often Should You Replace It?

General rule: every season or every 100 hours — whichever comes first.

For boats that run infrequently, this still applies. Sitting is just as hard on impellers as usage. If the impeller is more than a year old, replace it at the beginning of the season, not the end — so it doesn’t sit compressed all winter.

What Happens If You Wait Too Long

Overheating within minutes of startup
Melting of pump housings, hoses, or thermostat covers
Debris from shredded vanes causing hidden restrictions downstream
Localized engine damage — including warped heads and failed gaskets

We’ve seen engines destroyed in under 10 minutes because the impeller was assumed “good enough” for one more season.

Signs of Impeller Trouble

Reduced water flow from exhaust
Overheating at idle or low RPM
Increased engine temp despite good thermostat and coolant
Noisy or dry-sounding water pump during startup

Always Replace the Gasket and Key

Whenever we change an impeller, we replace the gasket and check the wear plate and shaft key. Reusing old parts can lead to leaks, misalignment, or premature failure of the new impeller.

Conclusion

If your impeller is older than a season, don’t inspect it — replace it. It’s a cheap part that protects a very expensive engine. Waiting until failure doesn’t just cost you a tow — it can cost you a block, a set of manifolds, or even a full repower. We treat impellers as scheduled maintenance, not repairs. You should too.