Onan generator raw water pump is eating impellers

My Onan generator raw water pump is eating impellers.

Most of the first year I owned my boat the generator and this pump worked fine. After the first service the generator water pump has been eating impellers.

During the service my mechanic noticed the generator seacock (cone valve) was hard to operate and he noted that I should have it replaced at the next haul out, but was able to return it to the open position after the service.

He replaced the generator raw water pump impeller and it pumped strong.

Within a couple hours of use, the impeller shredded. I assumed it was a faulty impeller and replaced it. That one shredded within an hour.

While replacing it again I checked the flow from the seacock, it is good. Once again the flow out the exhaust was good. This impeller lasted about an hour.

Suspecting a blockage on the raw water supply, even though the flow out the hose checked good, I installed a Groco sea strainer and installed a new impeller. This one lasted a couple of hours over 3 or 4 tests. Then it shredded.

Impeller 3 went up in smoke within 15 or 20 minutes.

Impeller 4 lasted several hours. I thought I had the problem behind me, and then it shredded. In each case the flow out the exhaust checked good upon start and during operation.

One other odd thing is that this pump doesn't have the cam to depress the blades. It appears to have been built that way, see diagram below.

The pump is mounted on the generator about 5 or 6 inches above the waterline so that it only has to pull about 8 inches of air out of the 1/2" hose when it starts.

I have back flushed the heat exchanger to verify that it isn't clogged with impeller debris. It flows clear and no debris came out. There were chunks of debris in the pump exit elbow and I cleared those each time.

The only 2 things I can think of to do are:

1. Add a cam lobe for the impeller chamber to improve flow.

2. Install a check valve in the supply lie to try and lessen the 8" of air in the line at start.

Pump needs to be rebuilt (preferably) or replaced, or both, giving you a spare. Give Depco Pump a call and discuss the options, they do a great job of providing either or both.

My first guess would be that the mechanic assembled something incorrectly, or left something out, and that same issues persists and is causing the impellers to self destruct.


I'm not sure what you mean by the "cam to depress the blades"? Does the pump assembly match the parts drawing, or is something missing?


A whole new pump might be a good next step. That would also give you something to compare against.


Hopefully someone familiar with this particular pump will chime in. I'm not. Is it an identifiable Jabsco, Johnson, or other pump?

I would send it to Depco for a rebuild with a note saying it is eating impellers. It looks like the inside of the housing is machined eccentrically accomplishing the same thing as a cam. My bet too is that something was left off or reassembled improperly after service.


David

Could be the bearing is loose?
Sending it in to Depco is going to be very pricey, likey $500 or more, the bearing is listed at $200 by itself.

Some people have used electric water pumps. likely that is the best idea.

My MCCK pump draws water up 2 feet.
My MCCK pump, I rebuilt the bearing (uses a typical wp 1 1/8 inch bearing shell, not like your double ball)with new balls from the hardware store, and seals from a bearing company called Hampton Rubber. I redesigned the D drive to a split design uses a common pin drive and the impeller prices went from( as high as )$80 to $5. (AliExpress)

https://www.pumpvendor.com/Onan_131-0257_parts.html

Buy a new pump. I carry a never used spare - consider the small genset pumps consumables. Any sign of leakage or issues pitch unless you're a hobbyist with the right on board tools, press and bench. Over heating the genset should be avoided strenuously.

Thanks for the replies.

If I can't figure this out, I think rebuild might be my only option.

Looking at the diagram there don't seem to be any parts missing as far as I and my mechanic have disassembled it.

The bearings feel perfectly smooth with no play. The shaft won't spin on its own when you spin it but I think that is because it has water-tight seals. There is no binding or wobble.

I played with an electric pump but I think I undersized it so the flow was less than I was happy with. I tried an 8 liter/minute pump visualizing that volume seemed fine, but in reality, the head in the exhaust side downstream of the muffler and the exhaust back pressure reduced the flow to far less than the stock pump. So now I am back to that.

David, sharp eye on the eccentric design, I missed that and couldn't figure out how it pumped, but you are right the shaft is built eccentric to the water chamber to create the pumping action.

Turns out I was just able to buy a used identical pump on eBay for $200.

A new pump prices at over $1500, so I'll see what I can figure out comparing the 2 and creating 1 good one... or if I am lucky 2 good ones.

f508 said:

Turns out I was just able to buy a used identical pump on eBay for $200.

A new pump prices at over $1500, so I'll see what I can figure out comparing the 2 and creating 1 good one... or if I am lucky 2 good ones.

Click to expand...

You should be able to just spin the shaft by hand, something is wrong inside it.
People are replacing coolant mech pumps with electric for large V8 engines today. There are pumps that can work. And somewhere I have read about people doing that with old Onan marine gens, maybe here on the forum.

Pump parts for Onan are absurdly priced, however you can get parts to work outside of Onan or marine sellers. The seal for my pump can sub in a a PS-163, but you have to reuse the ceramic washer. You can even buy a cheap water pump bearing for my pump, it is a standard wdth dimension, but have to machine the end to receive the SS impeller drive to be pressed in, and you can make one yourself out of a SS bolt.

f508 said:

Turns out I was just able to buy a used identical pump on eBay for $200.

A new pump prices at over $1500, so I'll see what I can figure out comparing the 2 and creating 1 good one... or if I am lucky 2 good ones.

Click to expand...

Wow! On my 12.5 Westerbeke the new raw water pump is less than $400. On my mains the new Jabscos cost me $750 apiece two years ago.

We have the same pump/ generator and have had similar woes when using those blue "run dry" impellers. It would spin the hub out of them because they are plastic. the hubs need to be brass (bronze?) . We also had a problem where I tried to use gasket goop instead of a paper gasket and the impeller bound on the housing without that little bit of extra space. Could this be the phenomenon that is causing your impellers to go?

Gabe and Em

Showing how I split drive shaft to use pin style impeller.
Cut a slot, then wedged it over, opening up to fit the new pin impeller. Works great. Cheap impellers they still make and lots of people sell. Pump OEM was designed to use D drive impellers, which are harder to buy and very expensive.

My repairs on Onan pumps.
https://photos.app.goo.gl/h4pWERsiKrvmKBZR9

sdowney717 said:

You should be able to just spin the shaft by hand, something is wrong inside it.
People are replacing coolant mech pumps with electric for large V8 engines today. There are pumps that can work. And somewhere I have read about people doing that with old Onan marine gens, maybe here on the forum.

Pump parts for Onan are absurdly priced, however you can get parts to work outside of Onan or marine sellers. The seal for my pump can sub in a a PS-163, but you have to reuse the ceramic washer. You can even buy a cheap water pump bearing for my pump, it is a standard width dimension, but have to machine the end to receive the SS impeller drive to be pressed in, and you can make one yourself out of a SS bolt.

Click to expand...

Yes, it easily spins by hand, but it doesn't keep spinning from the momentum. There is enough friction from the seals that it stops when I stop turning it.

Can you post a picture of the shredded impeller? Maybe someone can find a clue there.

f508 said:

Yes, it easily spins by hand, but it doesn't keep spinning from the momentum. There is enough friction from the seals that it stops when I stop turning it.

Click to expand...

Sounds ok then. You may find the new pump is no better. Could it be something in the line starts blocking the flow after the flow gets going? These pumps require continuous flow in and out, no partial blockages. Maybe you can prove the flow using city water pressure to flush the lines and heat exchanger and any filter. If the impeller gets hot from not enough water flow, it fragments into many pieces.

I once had a problem with grass and I have sucked up a jelly fish partially blocking the flow. Jelly fish got stuck in the through hull. Had to unscrew ball valve from hull to clean it out.

Other thing is using real old rubber impellers. They simply fall apart even with not much use if real old. Just like a tire cracks when rubber gets old.

sdowney717 said:

Showing how I split drive shaft to use pin style impeller.
Cut a slot, then wedged it over, opening up to fit the new pin impeller. Works great. Cheap impellers they still make and lots of people sell. Pump OEM was designed to use D drive impellers, which are harder to buy and very expensive.

My repairs on Onan pumps.
https://photos.app.goo.gl/h4pWERsiKrvmKBZR9

Click to expand...

That looks great. How did you find the outer diameter of your alternative impeller. Were the available impellers is a database?

For my 30 year old Onan 8KW, the impeller kit from Cummins is $178. I can order just the impeller for $38 but it doesn't include the little bronze thingy.....

f508 said:

That looks great. How did you find the outer diameter of your alternative impeller. Were the available impellers is a database?

Click to expand...

Somewhere, one site that sells them listed the sizes. External dimensions are identical to the D drive Onan impeller. It is a very common impeller. used on hundred of different gen pumps today
Jabsco 4528-0001 or equivalent.
Size is here 1 9/16" diameter by 3/4 " width is a very common sizing.

https://www.fisheriessupply.com/jabsco-impellers-pin-drive-4528-0001-p

I got 2 impellers for here, under $5, course out of stock now.
https://www.aliexpress.com/item/Imp...567.html?spm=a2g0s.9042311.0.0.29b24c4dt3sJAO

In stock $5
https://www.aliexpress.com/item/09-...lgo_pvid=577b6c39-6f19-42bf-961d-b0e8781a477d

$7, I can vouch for the Carbole impeller they were good quality for me.
https://www.ebay.com/itm/CarBole-Wa...01-Johnson-09-806B-Yanmar-Nanni-/252917479603

here is $13 Ebay seller, shows all the many engines using this impeller
https://www.ebay.com/itm/18-3079-Fl...VcXBheisjxnW+EKvuLTSR46qWTXlmalV6qLsWKy0XcQ==

Shredded impeller

jleonard said:

Can you post a picture of the shredded impeller? Maybe someone can find a clue there.

Click to expand...

The others all looked about like this too.

That is a pretty old pump. I wonder if the impellers you are getting are some old stock that has been warehoused for years. Old impellers tend to decay just sitting on the shelf.

Did all impellers come from one vendor?

I had trouble with a different machine not priming, and found that the new impellers were just slightly shorter than the old. Went to a different vendor and their impellers (same brand) were dimensionally correct. Priming problem went away..

One thing I would check is whether impeller is too tight in the axial direction. Maybe parts were swapped from a different pump, and impeller is getting squeezed. Try putting multiple gaskets in until you can blow through fittings, indicating impeller now has axial clearance, then take one gasket out.

It has been a long time since I have worked on what I gather is an old MDJ series machine, but then I had no recollection that they were impeller eaters. So I don't think there is anything wrong with the pump design.

And if they get hot, the blades fall off.
The new impeller should very slightly stick above the pump bore housing.
It is a rubbing fit on all sides in order to create a suction to draw in water.

Onan MDJA 3kw water pump, approx 1979

My water pump leaked at the seal behind the impeller. When I searched for the seal kit to fix the problem I found they no longer made it.

My work around was to bypass the pump entirely and use a 120volt electric pony pump. This pump would only come on when the generator generated electricity. Works like a charm.

The raw water goes into the electric pump and out to the heat exhanger.

cost $90

Ski in NC said:

That is a pretty old pump. I wonder if the impellers you are getting are some old stock that has been warehoused for years. Old impellers tend to decay just sitting on the shelf.

Did all impellers come from one vendor?

I had trouble with a different machine not priming, and found that the new impellers were just slightly shorter than the old. Went to a different vendor and their impellers (same brand) were dimensionally correct. Priming problem went away..

One thing I would check is whether impeller is too tight in the axial direction. Maybe parts were swapped from a different pump, and impeller is getting squeezed. Try putting multiple gaskets in until you can blow through fittings, indicating impeller now has axial clearance, then take one gasket out.


It has been a long time since I have worked on what I gather is an old MDJ series machine, but then I had no recollection that they were impeller eaters. So I don't think there is anything wrong with the pump design.

Click to expand...

I've been getting them from random vendors, whoever had them in stock, but they are old.

Good tip on the gaskets! Thanks.

grahamdouglass said:

My water pump leaked at the seal behind the impeller. When I searched for the seal kit to fix the problem I found they no longer made it.

My work around was to bypass the pump entirely and use a 120volt electric pony pump. This pump would only come on when the generator generated electricity. Works like a charm.

The raw water goes into the electric pump and out to the heat exhanger.

cost $90

Click to expand...

Would happen to remember the flow capacity for your electric pump?

The capacity of the pump was 300 gph. It was a "little Giant", if I remember correctly. A pretty cheap fix.

I recently bought a 5kw Northern lights with a sound shield. It came up on craigslist. Because I reconfigured my watertank in my lazzerette I have more room and can install a sound shield. I'll be removing my Onan and installing my Northern lights this March or April.

If the noisemaker is run hard the cooling water will be hotter.

Try using 2 paper gaskets instead of just 1 to allow for the heat.

FF said:

If the noisemaker is run hard the cooling water will be hotter.

Try using 2 paper gaskets instead of just 1 to allow for the heat.

Click to expand...

This pump is constantly passing raw water and doesn't get warm when the impeller is working. It is a little warm after the impeller fails, but that is after the fact.

Measuring the clearance shows the chamber that is about 1/16" wider than the impeller so I don't think there is any binding in there.

Could the intake hose from the seacock be collapsing internally and shutting off the water flow?

Or an air leak before the pump.

Do you lube the impeller with something like a heavy dose of liquid dish soap. That also helps the initial seal so it pulls water and primes faster. Sometimes it just takes a few seconds to many to cause an impeller to have trouble.

The intake hose isn't collapsing. It is pretty thick walled, and it is visible during operation.

On the install I apply the lubricant that comes with the impellers, it is a thick grease, but I imagine it dissipates with in 15 to 20 minutes.

The pump is mounted about 6"-8" above the waterline so there is a short dry period at each start, but that is at the starter rpm which is pretty slow and I imagine that the 1/2" waterline volume with the short run to the pump is sucked up before the engine starts and comes up to speed.

I've thought of putting a check valve on the hose, but since the generator will sit for a week or 2 at a time between starts I think even with a check valve the water would drop the few inches to the waterline.

My neighbor suggested that I check the flow through the heat exchanger in the normal direction. I haven't done that. I check it back-flushing for debris, but not forward, so I can try that.

In a few days the duplicate used pump will arrive from my eBay purchase and I can compare them and maybe something will stand out.

All good suggestions though. Thanks!