Generator losses raw water prime.

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Senior Member
Feb 1, 2020
Vessel Name
Vessel Make
Californian 45
I've got a Onan (Kabota I think) 8KW MDkD. Runs great but raw water won't self prime to the water pump. Water level is a couple inches below raw water pump. I have to run shop vac to discharge hose , at the mixing elbow. Start engine and when water flows into shop vac attach hose back to mixing elbow.
I've replaces raw water impeller twice. Can't find water leaks anywhere. Once primed it's fine. If I start the genny next day, it's fine. If I move locations and start genny next day... not fine. Shop vac again.
Is this a recently occurring problem or has it been long standing?
For first instance would be searching for loose hose connection, impeller plate seal failure or similar allowing air to seep in and break connection.
If long standing, rerouting supply hose to provide some type of loop to maintain sufficient water in pump to overcome on startup. This is the second option, would rule out all other possibilities before modifying.
I would second the pump as suspect. A worn cover or end plate, possibly a shaft seal, but that would likely show as a leak.
You have an air leak or a worn pump. Or both.
Attach your vacuum with the engine off and seacock closed and listen around the pump and hose connections for an air leak.
The pump may have a replaceable wear plate and the end plate can usually be reversed. Sometimes it can be the shaft seal.
Another option would be to create a high point in the raw water feed hose before the raw water pump. In essence, when air is draining the raw water pump, when air goes up to the peak in the raw water hose, the siphon effect is broken and water from the hose will collect in the raw water pump, keeping the impeller area full of water. The high point should only be a few inches higher than the raw water pump.

Probably a better solution would be a siphon break between the raw water strainer and the raw water pump on the generator.

All of my raw water cooled engines were setup where the impeller cavity remained full of water. Nothing good can happen from spinning an impeller in a dry space.

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