How to Replace your MerCruiser Water Pump Impeller

Step #1: Verify that the temperature gauge is reading accurately. Bad gauges are one of the most common causes of problems. In our opinion no boater should be without an IR-Thermometer on board the boat at all times. You can shoot the temperature of the thermostat housing which should tell you whether the temperature that the gauge on the dash is reading is accurate or not.

Mercruiser alpha 1 water pump kit


Step #2: Remove the thermostat housing and verify that nothing is stuck in the thermostat and preventing it from opening properly. Although thermostats can fail this is normally NOT the cause of most overheat related problems.

Step #3: Replace the Sea Water Pump Impeller

Impellers should be replaced every other year. It is important to realize that even with a new impeller just a small amount of “scoring” to the pump body can effect the output flow of the pump significantly. If you are replacing the impeller due to overheat issues versus routine maintenance, go ahead and replace the pump body as well.


Step #4: Verify there are no restrictions to the water flow going to the engine. Common causes of restriction are grass, weeds, and the remains from previous water pump impellers that come apart resulting from overheating. If your engine is equipped with an oil, power steering, fuel or engine cooler these will most likely be the areas for debris to become trapped. You should remove the hoses from the thermostat housing to the supply pump outlet as well as the pump inlet hose and backflush all the way out of the sterndrive to ensure there are not any restrictions.
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