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Engine Water Pump Service (MerCruiser)

Raw Water Pump Impellers should be replaced every year or two. Factory recommendations are usually every two years or 100 hours, whichever comes first. The prudent boater will change the impeller every year. The rubber impeller blades get worn and can fall apart causing overheating. Impellers also fail from age with little to no use.

Water Pump Styles Alpha One outdrives will typically have the water pump located in the drive. This pump can be difficult to replace for many boaters. The Alpha One Water Pump Kit is Mercury Number 46-96148A8. You should also replace the Alpha Pump Base too.

If you have an older R-Style pre-alpha then you will still use the Alpha One Water Pump Kit BUT you need a pre-alpha water pump base #46-57234A1.

If you have a newer Alpha Generation II drive (ser number 0D469859 and up or 1991 and newer) then you need a Alpha One Generation II water pump kit with the stainless housing #817275Q4. No base is required.

It is VERY important to know if you have an Alpha One or a pre-alpha "R" style drive. If you screw up and try to install the wrong water pump base you can destroy the outdrive.

Some Alpha Ones might not be using the pump in the drive but instead have elected to use a belt driven sea water pump. This is usually the case with closed cooling system equipped engines. Belt Driven Raw Water Pumps are also used on engines with Bravo drives. The belt driven MerCruiser Raw Water Pump now uses Kit # 46-807151A14.


MerCruiser MIE Inboards use a belt driven raw water pump. The older pumps used to have iron backing plates where the hoses would attach. For obvious reasons, this caused problems when the hose nipples rusted off. Mercury then updated the pump design to include a hard plastic rear plate assembly. The new plastic rear plate helped but the pump was still difficult to build because it had 3 gaskets, two wear plates, an impeller housing and the rear plate. Getting all this together without ripping the paper thin gaskets could be a bit of a trick.

Now if you order a new impeller housing you will get an upgrade kit instead. The old housing is no longer sold but it is for the better!

The Kit consists of the new ONE PIECE water pump housing. This kit eliminates all the cheesy paper gaskets and multiple pump assembly layers thus reducing the chance of leaks and making it much easier to assemble. The belt driven Inboard MerCruiser Raw Water Pump now uses update Kit # 46-807151A14. The Kit includes a new impeller, the one piece impeller housing, all the bolts and an o-ring.

Water pumps with the Fuel Pump Attached

Newer MerCruiser Bravo engines and MIE Inboards will have the fuel delivery pump attached to the raw water pump shaft housing. Make sure that you turn off any fuel valves before removal. Use a good line wrench to remove the fuel lines from the pump. While the pump is off and on the work bench for rebuilding, you might as well drain out the pump oil and refill it with clean MerCruiser High Performance Outdrive Oil.

Always grease up the impeller for those first few seconds that the pump is running dry. Check all the hoses and fuel lines for leaks. Make SURE you haven't reversed the "in" and "out" water lines. Don't forget to reopen your fuel valves!

Crank Mounted "Shaker" Pumps

Many non-MerCruiser engines such as Marine Power and Volvo will use a crankshaft mounted "shaker" pump.

Some of the older Johnson style and early Volvo pumps have become obsolete and you will have to buy the new style for a replacement.

It is important to try to center the pump during installation or it will wobble and shake really bad.

Circulating Pumps

Block mounted circulating pumps must be of the marine type. Automotive circulating pumps do not have bronze impellers, stainless backing plates or stainless steel shafts. They will fail in a few months.

The 454 or 7.4L GM Big Block Circulation Pumps are part number 850454 (gaskets are included).

The 350 cid, 5.7L, 305 cid, 5.0L GM Small Block Circulation Pumps are part number 850399 (gaskets are included).

MerCruiser 470 circulating pump

See Engine Cooling System Repair (MerCruiser Inline4) for information on the infamous coolant leak from the bottom front of the 470 aluminum block engine.