How to Service a Sterndrive Water Pump Impeller

Most engine makers recommend changing impellers every two years — sooner if operated in sandy or silty water. “If your engine runs warm at idle or slow speed, and then it runs cooler at higher speed, that’s a sign that the impeller needs to be changed,” says Jeff Fay, owner of Fay’s Boat Yard on New Hampshire’s Lake Winnipesaukee. Kyle Cosselman, a technician at Fay’s, shows us how on a Mercruiser Alpha drive. While not identical to servicing other drives, these steps provide a good picture of what the job entails.

1. Drain the gear lube. A few shavings on the magnetic plugs are normal, but milkiness, which indicates water in the gear lube, isn’t. Remove the rear anode to access the aft-most bolt. Remove two more bolts just ahead of the anode. Remove the nut from the front-most stud, and then use the nuts on the remaining two studs, plus gentle persuasion with a pry bar, to separate the drive halves.

2. Remove the copper water tube and its white plastic guide, as well as the drive-shaft O-ring, doughnut-shaped “slinger,” and impeller housing. Carefully pry the impeller from the housing. Don’t lose the key. Check the housing for heat warping and wear from sand. Also remove the pump-housing gaskets and metal wear plate. If you remove the lower pump housing, replace the O-ring beneath.

Mercruiser alpha 1 water pump kit
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