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How to Service a Sterndrive Water Pump Impeller

Keep your sterndrive running cool by servicing its water pump impeller.

September 1, 2016
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How to Service a Sterndrive Water Pump Impeller
Use this guide to properly service your sterndrive’s water pump impeller. Capt. Vincent Daniello

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.

How to Service a Sterndrive Water Pump Impeller
How to Service a Sterndrive Water Pump Impeller Capt. Vincent Daniello

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.

How to Service a Sterndrive Water Pump Impeller
How to Service a Sterndrive Water Pump Impeller Capt. Vincent Daniello

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.

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How to Service a Sterndrive Water Pump Impeller
How to Service a Sterndrive Water Pump Impeller Capt. Vincent Daniello

3. Clean all parts, then install the new gaskets, wear plate and the round, orange oil-passage seal. A dab of grease holds the key while installing the impeller, and a bit of liquid soap on the impeller lubricates it when first starting the engine. Rotate the splined shift shaft clockwise into forward gear.

How to Service a Sterndrive Water Pump Impeller
How to Service a Sterndrive Water Pump Impeller Capt. Vincent Daniello

4. Reinstall the impeller housing, drive-shaft slinger seal and drive-shaft O-ring. Insert the copper water-passage tube into the upper drive half and its white guide tube into the impeller housing.

“You’re doing a lot at once to get the two halves back together,” Cosselman warns. “Look through the exhaust cavity to see the water tube going into the white guide sleeve. Twist the drive shaft to align the splines as it goes into the upper gear set. And from below, make sure the splined [lower] shift shaft aligns with the [upper] shift fork, which should be pointed toward the bow.”

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How to Service a Sterndrive Water Pump Impeller
How to Service a Sterndrive Water Pump Impeller Capt. Vincent Daniello

5. Separate the upper and lower drive halves a bit to get the two forward nuts onto their studs. Reinstall the remaining front nut and three rear bolts. Reinstall the drain and vent plugs. While pressure-testing, rotate the drive shaft to test seals: It should hold 15 psi for 20 minutes. If so, fill with fresh gear lube from the bottom and install fresh anodes.

Quick Tip: The rear-most bolt hidden by the anode often corrodes, so an Allen key will no longer turn it. If so, either drill it out or slide a hacksaw blade between the upper and lower halves of the drive to cut it.

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