When to Replace Your Outboard Water Pump Impeller

The water pump impeller is a small rubber wheel in the lower unit that pushes cooling water up to the powerhead. When it wears out, the engine overheats. It is one of the cheapest parts on an outboard and one of the most expensive to ignore.

How often should it be replaced?

In Hawaiʻi we replace it every year or 200 hours, whichever comes first. Factory manuals do not agree. Of the 166 outboard service manuals we work from, 64 only say to inspect the impeller (or give no interval), 50 call for replacement at 300 hours or more, 33 at 200 hours, and 19 at 100 hours. Those intervals assume average conditions. Warm salt water, sand and long idle periods are not average, so where a manual says to inspect or wait longer, we replace it at the yearly service; where a manual says sooner, we follow the manual.

Why impellers fail here

• The vanes take a set. When the engine sits, the rubber vanes stay bent in one direction and stiffen. Warm water speeds this up.
• Sand and silt from shallow ramps and harbors wear the vanes and the pump housing.
• Running dry, even for a few seconds on a trailer without water, can burn the vanes.
• Age. Rubber hardens and cracks over time even with low hours.

Signs your impeller is worn

• A weak or sputtering tell-tale ("pee") stream.
• The overheat alarm, especially at higher RPM.
• Steam or very hot water from the tell-tale.

Pieces of a failed impeller can travel up into the cooling passages, so when we replace one we check that every vane is accounted for.

Find your engine's schedule

Our engine service guides show what the factory manual says about the water pump for your model, along with oil, gear oil and spark plug specs.