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Opening the Pump
Replacing a water pump impeller is usually easier than describing the procedure. Start by closing the intake seacock. Next spread an old towel beneath the pump to catch any small parts you drop or dislodge unexpectedly. On some engine installations, it can be easier to remove the pump entirely than to replace the impeller in place. This is an option to keep in mind if access to the face plate of your raw water pump is difficult.
With the seacock closed, remove the machine screws that hold the pump's cover plate in place. If these show signs of corrosion, plan to replace them now. Don't wait until one loses its slot or sheds its head.
If the cover is stuck to the pump body, pry it free, taking care not to distort it. The cover will be sealed with an O-ring or a paper gasket. The O-ring can be reused if it is still supple; a paper gasket normally must be replaced. Scoring on the inside of the cover plate reduces the pump's efficiency, so replace a scored plate. (Some pumps also have wear plates in the back of the pump chamber.)
Out with the Old
Removing the cover plate exposes the impeller. If it has a rubber plug in the center, pry that out of the shaft and set it aside. Now look at how the impeller is attached to the shaft. Most just slide over splines, a keyway, or flats on the shaft, but impellers are occasionally pinned to the shaft with a through-bolt or set screw. In this latter instance, remove the bolt or release the screw.
You may be able to get a grip on opposing vanes with thumb and forefinger and work the impeller out of the pump chamber. Otherwise pry it out with a pair of screwdrivers. It should come out without too much coaxing. Another method is to grip the hub between vanes with channel-lock pliers. Don't grip the vanes; pliers will tear the vanes and nix use of the old impeller as a spare.
Sometimes vanes have already torn off. If the impeller is missing vanes, be sure you can account for all the pieces. If they aren't in the pump body, they are probably in the outlet hose or the inlet side of the heat exchanger. Loose vanes are often the cause of unexplained and sometimes intermittent overheating. They are a particularly serious problem in a raw-water-cooled engine because they can get into the engines cooling passages, resulting in serious engine damage. Take the time to find and remove missing vanes.
Check at the base of each vane by bending it vigorously; if any show cracks, don't keep this impeller as a spare.
While the impeller is out, check the shaft for wobble. If it seems loose to you, you may need to dismantle the entire pump and replace the bearings and the seals. For the specifics of doing that, you should consult the service manual for your engine.
In with the New
Check the new impeller against the old one to make sure you have the right one. Grease the vanes and the wall of the pump chamber with petroleum jelly to provide lubrication for the few seconds the pump will run dry before priming. The grease also improves the seal of the vanes, helping the pump to prime more quickly.
If the shaft is keyed, make sure the key is in place, then slide the impeller onto the shaft. Fold the vanes to get the impeller inside the pump chamber. If you have difficulty here, tie the vanes folded until you get the impeller partially into the chamber. It doesn't really matter which way you bend the vanes; they will arrange themselves on the first revolution of the pump.
Push the impeller all the way home. Reinstall the through bolt, set screw, and/or the hub plug if your pump has any of these. Position the cover gasket or O-ring. If the gasket is paper, paint it with a flexible gasket sealant-both sides.
Screw the cover in place. Open the seacock. Start the engine and watch the exhaust to make sure the pump primes properly, then check the pump for leaks.
