How to Change the Water Pump Impeller of Outboard Engines

In this article, we will talk about what it is and how to change the impeller of the water pump of outboard engines.

All engines give off heat once the fuel starts to burn. This heat, if not dissipated, can lead to deformation or even melting of components.

Boat engines use a pump to draw in water and circulate it through the engine to cool it. Most pumps consist of 3 main components: the upper casing with an outlet that is inserted into the hose/pipe that carries water to the engine; the housing, an aluminum or stainless steel cylinder into which the impeller is inserted; the rubber impeller that rotates to draw in water; and the metal cover that closes off the lower part of the assembly.

Without a water pump, a boat engine can run for a few minutes at most until major malfunctions occur (which are implicitly very expensive to fix), so all boat engine owners need to pay special attention to the cooling system.

The most important part of the water pump is the impeller, a rubber rotor that is spun by the gear shaft to draw water in and push it into the cooling line. All manufacturers recommend checking and replacing it every 100 hours of engine operation (or more often if you are using the engine in salt water) or once a year.

The impeller is critical to the proper functioning of any boat engine. This is a worn part that can break (crack, melt, or lose its elasticity) due to impurities in the water and intensive use of the engine.

The actual change of the impeller

This is not a universal part. The shape and dimensions vary depending on the year and place of manufacture of the engine, series, or model. The safest and most accurate way to correctly determine which impeller you need is to open the water pump and, if the existing impeller is not stamped with an original code, measure the diameter of the shaft.

Caution. We say to measure the diameter of the shaft, the diameter of the metal housing, and its depth because the impeller itself wears out and the measurements taken are not accurate. It must be noted that the inner diameter of the metal casing must be smaller than the blade length of the impeller. The impeller must be set up with bent blades (counterclockwise) to allow the pump to draw in water, so the blades must be longer than the casing.

Installing the new one is an intuitive process (especially if you have already removed the old one). If you can't, just search on YouTube; there are many videos with instructions:

In conclusion, we would like to reiterate the importance of inspecting and replacing the impeller promptly. If you wait until it is destroyed, parts of it can get stuck in the cooling system and cause engine damage. It is much cheaper and healthier for the engine to be replaced regularly than to wait until problems occur.