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.

 

[Pmcsurf1]

Thanks for that. We've been doing a little experimenting while a a marina for the last three weeks. It appears we DO NOT lose prime while not moving. We've run the generator while at the dock and wait several days and it self primes. However, after running, slow mind you, at displacement speeds 6 to 8 knots, the pump losses it prime.
My intake clam shell is not a clam shell at all. It looks like a shower drain. Round with holes. I wonder if I switch to a clam shell it would keep positive pressure on the intake hose.

 

[Mischief Managed]

I think you need a new pump. They are not all that expensive. The Onan part number is 132-0277 which cross references to the Oberdorfer N202M-11, which is around 300 bucks. Onan prices are absurd. Here's one for $295: Oberdorfer FIP Pump Model# N202M-11. They typically do not include the gasket between the pump and the engine, The gasket part number is 185-2120 and it's hard to find in the parts manual.

 

[lwarden]

Do you have a strainer inline between the through hull and pump? Possible you could be getting an air leak there. You are right to think that when running you get a bit of suction at the through hull, but there would need to be a leak back through the pump and HX, etc. to create an issue. BTW, clamshell pickups are not recommended for generators with water lift mufflers as they can flood the engine via the exhaust.
Regardless, a good pump should easily be able to prime the line without damaging the impeller blades so likely this may be the main culprit.

 

[Greg QS]

Clam shell ( hi speed pickups) are not recommended for gensets. They will cause engine flooding if running while genset is not one. Especially with lift mufluers


You are over thinking this. Flex impeller pumps are self priming. They don't prime if the impeller is bad. Impellers go bad due to running dry. Your may have a back pressure issue ( restriction) or an air leak in the pump or the supply piping..

We can help you
put a clear piece of hose on the inlet side and look for air bubbles.
take the pump plate off and post pictures.
pull off the heat exchange cap inlet side and post pictures.
restrictions in the exhaust elbow will cause the problem you are having, Verry common also .

 

[Rackline]

Hopefully, you've resolved this issue by now! We were burning up impellers because of two problems. First, the strainer would not retain water after shutdown. The fix was to install a standpipe - a short piece of PVC pipe inserted inside the strainer to the intake side from the thru-hull to prevent the water from draining out. I've heard of some replacing the strainer with one where the intake is on the side instead of the bottom like ours. We still had overheating shutdowns and burned up several impellers. We cleaned the heat exchanger and installed a new exhaust elbow, but still had overheat shutdowns. We finally discovered an impeller fragment stuck in the hose between the strainer and the pump. I never thought of looking in the hose upstream for impeller fragments!

 

[Jetstream]

We had this problem with a brand new 12kva Onan generator and I found the easiest way to deal with it was just to close the through hull ball valve between generator operations to keep water in the intake line and pump. I just put a tag on the generator start switch to stop me from starting it with the valve closed.

I know of some people who have installed a strainer downstream of the pump to prevent impeller fragments getting into the heat exchanger.

 

[Comodave]

We had this problem with a brand new 12kva Onan generator and I found the easiest way to deal with it was just to close the through hull ball valve between generator operations to keep water in the intake line and pump. I just put a tag on the generator start switch to stop me from starting it with the valve closed.

I know of some people who have installed a strainer downstream of the pump to prevent impeller fragments getting into the heat exchanger.

You beat me to it. If you just want to have a work around this should do it. Or keep troubleshooting it.

 

[sunchaser]

Yes to a new pump And rebuild professionally the old one. These pumps are the life blood of any engine. Be sure to use OEM purchased from a reputable source, not an internet site claiming “original.”

 

[Taras]

I had this issue and found out that the replacement impeller I ordered was out of spec just enough that it allowed water to slip by the impeller when not in use and loose prime. I guess cheap chinese stuff..... Solution was to order a high quality american made impeller that was to the proper spec (even though the part numbers were the same)