How to Tell If Your Raw Water Pump Impeller Has Failed

The impeller in your raw water pump is one of the most important — and most fragile — components in your marine engine’s cooling system. It’s also one of the most commonly overlooked. If your boat is running hot, showing weak water flow, or steaming at the exhaust, a bad impeller is often the cause. Knowing the symptoms and how to check can save your engine from serious heat damage.

What the Impeller Does

The impeller is a flexible rubber vane mounted inside the raw water pump housing. As it spins, it draws seawater from the intake and forces it through the engine’s cooling passages. It’s responsible for all raw water movement through your engine. If it’s damaged, worn, or missing vanes, cooling stops — sometimes instantly.

Signs Your Impeller Has Failed

No water from the exhaust outlet: This is the most obvious and immediate symptom. If the impeller can’t push water, the exhaust stays dry.
Rapid overheating at idle: The engine may heat up within minutes of starting, especially if running in the water without forward motion.
Squealing or whining noise from the pump area: This may indicate a dry impeller running without lubrication.
Rubber smell or melted impeller pieces in the strainer: If the pump runs dry, the impeller overheats and starts to break down internally.
Engine overheats despite clear intake and working thermostat: This narrows the issue to the impeller itself.

How to Confirm Impeller Failure

Shut off the engine and allow it to cool completely.
Locate the raw water pump, usually on the front or side of the engine.
Remove the pump cover plate or end cap.
Inspect the impeller for missing vanes, cracks, or a melted appearance.
If any vanes are missing, check downstream (in the oil cooler, heat exchanger, or hoses) — broken pieces often get lodged there.

If the impeller is worn, cracked, or flattened, replace it. Always install a new gasket or O-ring and lubricate the new impeller with water or dish soap before startup.

Common Causes of Impeller Failure

Running the engine dry, even for 15 to 30 seconds
Age — rubber hardens over time, even if not used
Overheating from intake blockage or suction leaks
Incorrect storage without removing and inspecting the part

How Often Should You Replace It?

Impellers should be replaced every 12 months in saltwater or every 2 seasons in freshwater. Even if they appear intact, rubber fatigue or loss of flexibility can reduce efficiency. It’s cheap insurance compared to the cost of overheating.

Conclusion

A failed impeller can take down your entire cooling system — and it doesn’t take long. If you’re seeing no water from the exhaust, overheating at idle, or signs of dry running, check the impeller before anything else. It’s a small part with big consequences. Regular inspection and yearly replacement are the best ways to keep your engine safe, cool, and running strong.