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The trick to inserting a new water pump impeller

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What is the issue?
Flexible rubber impeller pumps are the most common mechanism to cool yacht engines. However, the rubber impeller wears and degrades requiring regular replacement, typically annually. This can prove to be difficult, particularly so for larger pumps where the vanes can be disagreeably stiff or when the pump housing is difficult to access in the confines of an engine compartment.

Why address this?
A failed impeller can overheat the engine. Making the impeller servicing an easier task means it is more likely to be attended to. A smooth implementation could make the operation of changing a failed impeller more agreeable whilst at sea.

How to address this?
The problem with an impeller replacement usually centres around mounting the new replacement impeller with its stiff blades into the housing. Using a cable tie at this point, to precompress the blades and reduce the replacements diameter, should help to make the new impeller's insertion much easier. Just follow these steps:

  • 1. Thoroughly clean out the impeller housing with a clean cloth and apply a good coat of petroleum jelly (Vaseline). If petroleum jelly is not available applying a lavish amount of dishwashing detergent will do the job.

  • 2. Cover the replacement impeller with petroleum jelly. This helps ease the impellor in but also acts as initial lubrication as the pump will not be full of water during initial startup.

  • 3. Tie an electrical cable-tie around the impeller.

  • 4. Tighten the cable-tie down until it draws in the impeller vanes into a contracted shape but not overly tight that it is difficult to remove.

  • 5. Press the constricted impeller into the pump housing.

  • 6. As it slides into place the cable tie will come against the outer edge of the housing. Whilst pushing the impeller in, pull the cable tie off with pliers if need be, releasing the vanes inside the pump housing. When the cable tie comes off the impeller will then happily push home.

  • 7. Replace the cover. Some are shaped so the orientation is obvious. But if you happen to have a perfectly round cover, and it is not obvious, look for any writing or branding on it. The text will normally be the right way up showing how it is orientated. Failing that you should be able see the shadowing of the cam on the cover and figure out the orientation.

  • 8. Tighten the cover in a cross orientation - like you would the wheel of a car. Go gentle, they should be just snug as the screws are tightening into a bronze housing. This is not that strong a material and it is easy to strip out the threads by overtightening.

  • 9. Once the engine turns over it will automatically turn the impeller vanes into the correct position so there is no need to try match the original alignment.

Impeller constricted by a cable tie
Photo: Michael Harpur

Getting the impeller in is usually the challenging part of the operation. However, sometimes you can have difficulty removing the original impeller. The best removal technique for a jammed impeller is to use a pair of pliers. Grip on a vane from either side with both pliers and pull it out.

It is essential not to prise tools off the edge of the housing in order to lever out the impeller. The housing is usually made of a soft bronze material that is easily deformed. Any damage to the edge could lead to an improper seal causing an air intrusion or a leak. This could lead to it sucking in air when the boat is running causing an airlock, or leaking water when it is not in use.

With thanks to:
Michael Harpur, Yacht Obsession.

Volvo Penta sea water pump impeller replacement

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