Common Causes of Outboard Impeller Failure

The impeller is one of the smallest parts of an outboard engine's cooling system, yet it’s also one that can fail at the most inopportune times and cause the largest amount of damage, including total engine failure.  

Outboard water pump impeller failure

An engine’s water pump is basically useless without an impeller, as this little component is what generates the centrifugal force to pass cooling liquid through the engine. As small and simple as they are, impellers often cause the largest amount of outboard engine problems.

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What Does The Water Pump Impeller Do?

The impeller is the water pump component that distributes cooling liquid (in most cases water) throughout an outboard engine’s cooling system. Impellers have rotating vanes that circulate water (or coolant) through the engine. When the impeller fails, the engine overheats. Also, it’s not uncommon for pieces to break off of a worn impeller and get sucked into the engine, which wreaks havoc on the entire motor.

water pump impeller failure common causes

What Does a Broken Impeller Sound Like?

If you hear a grinding or rattling sound coming from your boat’s engine, it could mean the water pump impeller is broken. Another sound a broken impeller makes is a screeching noise, possibly due to water pump bearings going bad.

Besides unusual noises like grinding, rattling and screeching, other signs that an impeller has gone bad include water pump leakage, reduced water pump flow, slow engine re-priming and engine overheating.

What Can Damage an Impeller?

Catching potential water pump impeller failure early on will save you a lot of time and money in engine repairs. Impellers can fail for any number of reasons, including running dry or sitting inactive for too long. Here are the three main causes of water pump impeller failure.


Cavitation happens when there’s a pressure difference on the water pump body or the impeller itself. If the pressure drops suddenly, liquid can turn into vapor and cause cavitation, which can then lead to disintegration of the impeller.


Erosion happens when hard particles such as sand come into contact with the impeller. Over time, erosion will wear down the impeller, causing it to become brittle and eventually break apart and fail. 


Corrosion happens when the pH of the water running through the water pump is too low, turning it acidic and making it gradually eat away at the impeller. 

Cavitation, corrosion and erosion all impair an impeller’s ability to circulate water through the cooling system effectively, and in severe cases can lead to total water pump failure. 

How Often Should You Change an Outboard Impeller?

Water pump impellers should be inspected at least once a year for cracks, rotting and deformities, and for broken or missing blades. You should follow the manufacturer's recommendations for how often to replace an impeller, but a good rule of thumb is to replace it every 3 years or after 100 hours of use. Thankfully, water pump impellers are inexpensive and easy to replace, and you should always carry a spare or three on your boat just in case an impeller fails. has produced many water pump repair videos for various makes and models. If you need help replacing a water pump impeller for your Mercury or Yamaha outboards, check out our video playlists below:

Mercury Outboard Water Pump Repairs Video Playlist

Yamaha Outboard Water Pump Repairs Video Playlist




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