What does the impeller do?
The impeller is a rubber wheel in the raw water pump. It pumps cooling water into the engine. If it fails, the engine overheats quickly.
Signs of a worn impeller
- The cooling water stream from the tell-tale hole is weaker than usual, uneven or almost gone at higher revs.
- The engine runs fine at idle but gets hot at high revs over longer periods – the pump can't keep up when flow demand increases.
- Temperature alarm or rising temperature.
- Cracked, stiff or broken vanes when inspected.
- Vanes that have been bent for a long time and don't straighten out.
How often?
The replacement interval is given in your engine's owner's manual, usually as a number of hours or years, and manufacturers' recommendations vary. Always follow the engine manufacturer's manual.
Don't just count hours. Rubber ages chemically even when the engine isn't running – an impeller left unused over the winter can harden without a single running hour. Sand, lime and grit in the water also wear down the vanes, more so in murky coastal water than in clear fresh water.
The impeller needs replacing at regular intervals to keep the engine from overheating.
How to replace it – in brief
- Close the seacock.
- Unscrew the pump cover.
- Pull out the impeller with impeller pliers or two screwdrivers.
- Find all loose vane fragments – they can clog the cooling system.
- Lubricate the new impeller with a little dish soap or glycerin and press it in with the vanes bent the right way.
- Fit a new gasket or O-ring and screw the cover back on.
- Open the seacock, start the engine and check the water flow.
Have at home
- A spare impeller with the right part number for your pump.
- Gasket or O-ring for the cover.
- The engine's model and serial number so you can order the right parts.
Check after replacing
- Start the engine and check that the cooling water stream appears immediately and is steady.
- Keep an eye on the temperature during the first trip.
- Check that the pump cover isn't leaking.
- Note the date and engine hours of the change, so you know when the next one is due.
It's a good idea to change the impeller during winterization or spring commissioning, while the boat is on land anyway.
Hard to reach?
On some engines the pump is hard to get at or sits in the drive. Then it can pay to have a workshop do the job during a service.
Frequently asked questions
How often should you change the impeller?
Follow the interval in the engine's owner's manual. Many owners replace it more often as a precaution, since the rubber ages even when the engine isn't used.
What happens if the impeller fails?
The cooling water stops flowing and the engine overheats. Loose vane fragments can also get stuck in the cooling system and must be found.
How do I know which impeller I need?
Start from the engine's model and serial number or the pump's markings, and compare with the manufacturer's spare parts catalogue.
Can the impeller go bad even if the engine has barely been run?
Yes. Rubber ages even when the engine isn't running, and an impeller left unused over the winter may have hardened.
Sources
- Impellrar för båtmotorer, Biltema biltema.se
- Support och instruktionsböcker, Volvo Penta volvopenta.com