How to Replace the Water Pump Impeller

Tilt the outboard down to vertical and support the boat on a trailer or stands. Shift into neutral. Note or photograph the shift position and the position of the trim tab (mark it with a marker before removing it).

Wedge a block of wood between a prop blade and the anti-ventilation plate, remove the prop nut and hardware, and slide the prop off. Note the order of the thrust washer and spacer.

Remove the bottom drain screw and top vent screw and drain the gear lube into a clean pan. Check it for water or metal while you are here (see the lower unit oil guide).

Some outboards need the shift rod disconnected before the gearcase can come off; others just pull apart in neutral. Follow your manual. On some models you also disconnect a speedometer hose.

Remove the bolts or nuts holding the gearcase to the midsection (often four to six, some hidden behind the trim tab). Use a box-end wrench so you do not round them off. The gearcase usually sticks; tap the rear of it gently with a soft mallet. Never hit the anti-ventilation plate edges, which break easily.

Pull the gearcase straight down, keeping the drive shaft vertical, and set it upright in a safe place.

Slide off any seal or grommet on top of the pump. Remove the pump housing bolts and lift the housing up the drive shaft. The impeller usually comes with it. Remove the impeller, the drive key (a small pin or flat key on the shaft) and the wear plate and gaskets underneath.

• Impeller: missing or cracked vanes? Find every missing piece. Broken bits can lodge in cooling passages and cause overheating later.
• Housing: deep grooves, burned plastic or a warped cup mean replace the housing (most kits include it).
• Wear plate: a deep groove means replace it.

Where do missing vane pieces go? Water carries them downstream, so search in the direction of flow: the pump housing outlet, the water tube or its grommet, and then the first hose or fitting after the pump. On engine-mounted pumps, check the outlet hose and whatever it feeds first (often a heat exchanger, oil cooler or thermostat housing). Count the vanes on the old impeller so you know how much you are looking for. If pieces are still missing and you cannot reach them, have a shop find them before they block a passage.

Scrape off all old gasket material and sealant with a plastic scraper or razor blade, then clean with fine emery cloth and a rag. Leaks here let air into the pump and reduce flow.

Fit the new gaskets and wear plate from the kit in the same orientation as the old ones. Gaskets are often not symmetrical; if holes do not line up, flip it. Use sealant only where your manual says.

Place the new drive key on the flat of the shaft (a dab of grease holds it). Coat the impeller vanes with glycerin or mild dish soap, not oil-based products, which can attack the rubber. Slide the impeller down so its slot lines up with the key.

Lower the housing over the impeller while turning the drive shaft clockwise (the normal direction of rotation) so the vanes fold the right way. Never turn the shaft backward. Install the housing bolts and tighten evenly to spec. Fit the top seal or grommet.

Lightly grease the drive shaft splines with the lube your manual specifies. Lift the gearcase into place, aligning the water tube with the pump grommet and the drive shaft with the crankshaft splines (turning the flywheel slightly by hand, clockwise only, helps them mesh). Reconnect the shift linkage, install the bolts to the torque in your manual, reset the trim tab to your mark, and refill the gear lube from the bottom. Reinstall the prop.

Before starting, check shifting: with the engine off, confirm it shifts into forward, neutral and reverse correctly.

Close the seacock if equipped. Remove the cover plate screws (keep them in a cup) and pull the cover. Pull the impeller out with an impeller puller or two screwdrivers placed opposite each other, being careful not to gouge the housing. Check for missing vanes. Clean the housing and plate, lubricate the new impeller with glycerin, push it in while turning it in the pump's direction of rotation, and install a new gasket or O-ring and the cover. Tighten the screws evenly. Some pumps are easier to service off the engine. Reopen the seacock before starting.

Run the engine on flush muffs or in a test tank (a test tank shows the pump's true suction best). Do not use the built-in flush port for a running test; on most outboards it is for engine-off flushing only, and even on models allowed to idle on it, the port usually feeds water in past the pump, so it can't show whether the new impeller pulls water. Within a few seconds you should see a strong, steady telltale stream (outboard) or water from the exhaust (sterndrive/inboard). Watch the temperature, check for leaks, and log the date and hours.