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Old 02-09-2019, 10:00 AM   #1
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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.

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Old 02-09-2019, 10:09 AM   #2
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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.
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Old 02-09-2019, 10:19 AM   #3
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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?
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Old 02-09-2019, 10:56 AM   #4
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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.


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Old 02-09-2019, 11:25 AM   #5
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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
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Old 02-09-2019, 11:45 AM   #6
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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.
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Old 02-09-2019, 12:42 PM   #7
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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.



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Old 02-09-2019, 12:52 PM   #8
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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.
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Old 02-09-2019, 01:33 PM   #9
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Quote:
Originally Posted by f508 View Post
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.
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.
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Old 02-09-2019, 01:45 PM   #10
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Quote:
Originally Posted by f508 View Post
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.
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.
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Old 02-09-2019, 02:03 PM   #11
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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?

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Old 02-09-2019, 02:40 PM   #12
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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
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Old 02-09-2019, 03:05 PM   #13
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Quote:
Originally Posted by sdowney717 View Post
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.

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.
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Old 02-09-2019, 03:13 PM   #14
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Can you post a picture of the shredded impeller? Maybe someone can find a clue there.
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Old 02-09-2019, 03:18 PM   #15
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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.
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.
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Old 02-09-2019, 03:21 PM   #16
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Quote:
Originally Posted by sdowney717 View Post
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
That looks great. How did you find the outer diameter of your alternative impeller. Were the available impellers is a database?
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Old 02-09-2019, 03:23 PM   #17
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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.....
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Old 02-09-2019, 03:28 PM   #18
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That looks great. How did you find the outer diameter of your alternative impeller. Were the available impellers is a database?
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/jabs...ve-4528-0001-p

I got 2 impellers for here, under $5, course out of stock now.
https://www.aliexpress.com/item/Impe...29b24c4dt3sJAO

In stock $5
https://www.aliexpress.com/item/09-8...d-b0e8781a477d

$7, I can vouch for the Carbole impeller they were good quality for me.
https://www.ebay.com/itm/CarBole-Wat...-/252917479603

here is $13 Ebay seller, shows all the many engines using this impeller
https://www.ebay.com/itm/18-3079-Fle...sWKy0XcQ%3D%3D
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Old 02-09-2019, 03:29 PM   #19
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Shredded impeller

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


The others all looked about like this too.
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Old 02-09-2019, 03:50 PM   #20
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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.
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