The rubber impeller in your raw-water pump is one of the cheapest parts on your boat, and one of the most important. When it fails, cooling water stops, the engine overheats, and a good day on the water turns into a tow home. The good news: replacing an impeller is a job most owners can do themselves with basic tools and an hour or two. This guide covers the process for a typical outboard or inboard raw-water pump. Always check your engine's workshop manual for model-specific steps and torque figures.
Why impellers fail
Flexible impellers are made from neoprene or nitrile rubber. Over time the vanes take a "set" (they stay bent in one direction), harden, crack and lose their seal against the pump housing. Running the engine dry, even for a few seconds, can tear vanes off completely. Sand, silt and long lay-ups make it worse. Most manufacturers recommend replacing the impeller every one to two seasons or every 100–300 engine hours, whichever comes first. If you've just bought a used boat and don't know when it was last done, do it now.
Signs your impeller needs replacing
- A weak or intermittent tell-tale (pee stream) on an outboard
- Rising temperature gauge readings or overheat alarms
- Steam or very hot water from the exhaust on an inboard
- Rubber fragments found in the cooling system or strainer
What you'll need
- The correct replacement impeller or impeller kit for your pump. For example, the JMP 9000K impeller kit (Jabsco 18786-0001 replacement) or the JMP 7136K impeller kit (Johnson 09-812B / Volvo 22307636 replacement). Match the part number on your old impeller or pump.
- A new cover gasket or O-ring. Never reuse the old one. The 01-42445 pump cover gasket suits many common pumps.
- An impeller puller makes removal much easier and protects the housing: see the seawater pump impeller puller (2.5"–4.5") or the impeller puller for Johnson 09-821B.
- Screwdrivers or sockets, a small pick, silicone or glycerine-based lubricant, and a torch.
Step-by-step: replacing a raw-water pump impeller
1. Make it safe
Turn off the engine and remove the key or kill cord. Close the seacock on an inboard. Let a hot engine cool before you start.
2. Access the pump
On most inboards and sterndrives the raw-water pump is on the front or side of the engine, with a cover plate held by a few screws. On many outboards the pump sits in the lower unit, so you'll need to drop the gearcase. If you aren't confident with that, an outboard pump job is a good one for a mechanic, but the parts list stays the same.
3. Remove the cover plate
Undo the cover screws evenly and lift the plate off. Note the position of any cam or wear plate inside the housing.
4. Pull the old impeller
Slide a puller over the impeller hub and draw it straight out. Avoid levering against the housing with screwdrivers, because scoring the housing reduces pump efficiency. As it comes out, note which way the vanes were bent. That shows the direction of rotation.
5. Inspect everything
Count the vanes on the old impeller. If any are missing, find them. They usually lodge in the heat-exchanger inlet, oil cooler or hose fittings downstream and will block flow. Check the housing and cover plate for grooves or scoring and replace a badly worn wear plate.
6. Fit the new impeller
Lightly lubricate the vanes and inside the housing with a recommended lubricant. Don't use petroleum grease, as it degrades rubber. Bend the vanes in the direction of rotation and push the impeller onto the shaft with a slight twisting motion, lining up the drive key, spline or screw slot.
7. Refit the cover
Fit the new gasket or O-ring, replace the cover and tighten the screws evenly in a cross pattern. Don't overtighten. Too much force can distort the plate. If your pump cover has a paper gasket that needs sealing, a thin smear of Permatex 22071 water pump & thermostat RTV can help where the manufacturer allows it.
8. Test run
Open the seacock, start the engine and check straight away for a strong flow of cooling water. Look for leaks around the cover. Keep an eye on the temperature gauge for the first run.
Tips for longer impeller life
- Never run the engine without water, including on muffs or a flushing attachment with the tap off.
- Flush with fresh water after every saltwater trip.
- Carry a spare impeller and gasket on board. They're small, cheap and can save your trip.
- If the boat will be laid up for months, some owners remove the impeller to stop the vanes taking a set.
For Yanmar owners, a complete rebuild is often the better choice. The Yanmar 2GM/3GM/2YM/3YM seawater pump kit covers more than the impeller.
Find the right impeller
Big Burjos Marine stocks impellers and pump kits for Jabsco, Johnson, Sherwood, Volvo Penta, Yanmar, Mercruiser and more, shipped Australia-wide from our Coomera, QLD warehouse. Not sure which part fits your engine? Email us your engine model and the part number from your old impeller and we'll help you match it.