Generator losses raw water prime.

[Pmcsurf1]

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.

 

[Turtle Blues]

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.

 

[Bmarler]

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.

 

[Lepke]

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.

 

[O C Diver]

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.

Ted

 

[rnanaples]

Sounds like a weak pump. Is there a vented loop in there somewhere that could be worn out?

 

[Dan3281]

I had a similar issue and was due to a worn impeller backing plate. Does yours have wear grooves? Replacement plates are available and as a short term fix, just flipped mine over. As others have already said, a cut or misplaced o ring can also be a cause.

 

[joezzz]

I had the same problem with my Onan generator this past winter.

best to put in a new one, and then have the old one re built. As the wear could be in the seal, the bushing or the shaft.

 

[Greg QS]

These pumps suck. I mean really suck well.
When impeller pumps don't prime check.---

impeller. Make sure you lube liberally. Do not install dry . you can destroy the impeller in seconds dry.
back pressure a blockage down stream, cloged hx or ,exhaust elbow ,kink in exhaust hose
air leak in pump / seals
worn cam,
worn backing plate.
Look for drips on the inlet side from the hull. If it is leaking it will suck air and not prime properly .
Are you using the correct impeller? Make sure it is the correct blade count, thickness, and diameter.
Are you using the correct gasket. Some people will use a homemade gasket that is too thick. This can stop the pump from priming. This creates the affect of a worn backing plate. The impeller must touch the back and the front of housing to work properly.

 

[HopCar]

It’s already been said, worn cam, worn backing plate and worn cover. The cover and backing plate can often be reused just by flipping them over. Be sure to record the numbers on the front of the cover before flipping it.

 

[Cartouche]

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.

it,s either the pump the backing plate or if you are using a Chinese's knock off impeller they will not seal properly

 

[Boatcrazy]

We have a 1989 Onan Genset, 6.5kva, with the same problem. After lots of research and various mechs, several new impellers and cover plates, etc, etc, the problem was IDed as a worn-out and mis-shaped pump body. We bought a new Onan pump (not cheap!!!), installed it, and have not had an issue since.