Onan generator raw water pump is eating impellers

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

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What can happen is that the inner layer collapses and may not be visable from the outside.

jleonard said:

What can happen is that the inner layer collapses and may not be visable from the outside.

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Oh, I wasn't aware of that. I'll replace that hose then. That segment is only a couple of feet long.

Thanks!

I decided to go ahead and remove the galling from the failed impellers in the interior of the chamber. I cleaned the inside up and then turned the mounting surface down to maintain about 3/16ths clearance.

Replace it with a March 120VAC AC raw water pump and be done with it.

I have the same pump as yours on my Onan mdje 7.5 kw. It tends to vapor lock when I raise the sails in moderate winds or seas. I always check for water flow throug the exhaust outlet whenever I start the gen set. When I run out of spare empellars, I will install a March pump—many have done it.

For $1500 I would be looking at the March pump solution too!
It would be pretty easy to hardwire the pump to the genny connections box with its own circuit breaker.

I was going to go down the electric pump road. I could mount an electric above the waterline. I am hesitant to introduce a plastic pump in a raw water circuit, so I thought I would take a good run at getting this to work.

The really odd thing is that it worked great, until it stopped working. So I keep hoping that I can make it work.

electric pump work around

The electric pump I used to bypass the rawwater pump was steel, pretty robust. Little giant, 300 gph, 120v

grahamdouglass said:

The electric pump I used to bypass the rawwater pump was steel, pretty robust. Little giant, 300 gph, 120v

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I see. Having done a little more searching I think I found the one you bought. Very nice, and a good solution. If this next round of repairs doesn't do it, I think I'll fit one.

Thanks.

"If this next round of repairs doesn't do it, I think I'll fit one."

Be sure to be able to switch it OFF , if ever bleeding the engine , as a running pump and stopped engine can fill the exhaust system, then the engine..

If you go electric, just tie the pump leads directly to gen output through a breaker. That will prevent pump from running unless gennie is running.

This will prevent the flooding problem FF described above.

I like the idea of wiring into the 120v output to sync the pump operation with the generator operation.

I hadn't thought through the consequences of a running pump with the generator off. My exhaust routing would fill the exhaust manifold, the muffler and then the cylinders and depending upon the exhaust and intake valve positions possibly the air intake and then the engine room. Pulling CB protected power from the generator output would seem to guard against that.

Thanks for the tip.

Any electric raw-water pump installed should be wired directly to the generator head. Don’t buy a cal pump—March is a quality pump; however it is also a centrifical pump. Make sure it is mounted below the waterline.

Check that your bearing isn't frozen. I had an alternator belt that disintegrated. I replaced it and it blew apart and took it back to the parts store. The guy replaced it, but he was not happy. Put the new one on and the same thing. The second belt, definitely not happy and said this is it. Put the second one on and watched, sure enough the pulley was frozen. I replaced the bearing in the pulley and bought the new belt elsewhere. It's worth a try. I went to a bearing distributor and they pressed in a replacement no charge. Nice....

Looking at the picture your failed impeller, it seems that the centre hub worn on one side more than the other ? . Is it just an illusion or is it that the impeller is not running true to the end plates.

BeeJay said:

Looking at the picture your failed impeller, it seems that the centre hub worn on one side more than the other ? . Is it just an illusion or is it that the impeller is not running true to the end plates.

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It must be how the light is hitting it. The impeller was more or less symmetrically destroyed.

Onan raw water pump

I have a new onan raw water pump for sale if you are interested. I bought (a hellishly expensive one from Onan) to replace mine. Then something else died on the genset and I had to replace the whole unit. So I have a pump with only a couple of hours on it
Contact me if you are interested.
Rick

Rickschnurr said:

I have a new onan raw water pump for sale if you are interested. I bought (a hellishly expensive one from Onan) to replace mine. Then something else died on the genset and I had to replace the whole unit. So I have a pump with only a couple of hours on it
Contact me if you are interested.
Rick

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Thanks, but I am good. I have mine that I have cleaned up and then another used one that I bought on eBay. If I can't get it to work with all that, I am going to switch to an electric pump.

f508: In your original posting you mentioned that the tapered sea cock needed to be replaced. They are stone simple to rebuild/overhaul and considered by many to be superior to the more modern ball valve replacements. There was a great article in Good Old Boat about a year ago and several google postings on the topic - even a youtube video. The recommendation is to exercise the sea cock periodically to keep it from becoming frozen once it is overhauled. You will need to haul the boat but, if it were my boat, (the 48 DeFever or any DeFever is on my dream list), I would sure inspect and service before replace. Good luck!

Are you sure that the impeller is mounted correctly and that the length and diameter are correct? Looks like you are getting too much friction on the impeller and it’s overheating and self destructing. Have you tried a slightly smaller impeller? May allow some water bypass but unlikely to overheat and self destruct. May be worth a try.

Also, have you check the interior surface of the pump for burrs etc as well as the end plate for friction and fit?

Good luck. Hope you eventually post the identifcation of the problem.

Unfortunately I wasn't there for the original service. I have been ordering the replacement part number listed in the manual for the pump number that is stamped on the pump. The clearance seemed tight to me so I put the pump chamber on a lathe and turned it a bit larger. I also increased the side clearance. I also polished the interior. There were some casting defects that hadn't been milled out during manufacture. If this impeller fails, I have one more that I will try trimming before installation.

I certainly will post a solution if I find one. My bail out is to install a modern electric pump if this next round of ideas doesn't do it.

Exctyengr said:

f508: In your original posting you mentioned that the tapered sea cock needed to be replaced. They are stone simple to rebuild/overhaul and considered by many to be superior to the more modern ball valve replacements. There was a great article in Good Old Boat about a year ago and several google postings on the topic - even a youtube video. The recommendation is to exercise the sea cock periodically to keep it from becoming frozen once it is overhauled. You will need to haul the boat but, if it were my boat, (the 48 DeFever or any DeFever is on my dream list), I would sure inspect and service before replace. Good luck!

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I'll look into that. I think I have another year before I'll need to do the bottom, so I'll get that figured out before I pull her out. I think it would be pretty expensive to cut out the existing valve and reglass in a new one, so I'll rebuild if I can.

Did you ever call Depco and talk it over with them and get a price on a rebuild or new? They are extremely expert and very helpful. Or just get a rebuild kit from them or whatever parts they recommend. $1500 doesn't sound right for a new one. A pump for my 20KW Onan MDL4 was less than half that last I checked, which admittedly was a few years ago.

http://online.anyflip.com/ucrc/jfuq/mobile/index.html#p=134


800-446-1656 and ask for someone in marine sales.

Raw water pumps have two key failure points. The bearings, with their tell tale rusty leak on the weep holes, and infrequently, impellers.

If bearings fail, I've learned never to have a pump rebuilt. I always buy a replacement pump. Rebuilt pumps never seem to last and it's just not worth the hassle.

Impellor-wise, I change mine at least every two years. Given the issues you've had, something is wrong with the pump. In my book, that means change it.

And yes, I carry replacement pumps, but all are new pumps.

We were told about some issues with Onan's that where mounted well above the waterline like ours so when we got the boat so I just closed the seacock to the generator immediately after shut down and open it right before start every time. Put a sign over the start switch to remind me before every start and never had a problem with it. Changed the impeller every 200hrs, used plenty of Dow Corning Compound 4 on it every time and the old one used to look like new when it came out.

Jetstream said:

We were told about some issues with Onan's that where mounted well above the waterline like ours so when we got the boat so I just closed the seacock to the generator immediately after shut down and open it right before start every time. Put a sign over the start switch to remind me before every start and never had a problem with it. Changed the impeller every 200hrs, used plenty of Dow Corning Compound 4 on it every time and the old one used to look like new when it came out.

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That is a great idea!

Unfortunately for me, I don't think it will solve my problem because mine seems to start pumping fine, and I verify water is coming out of the exhaust, and then later while running, the impeller shreds.

f508 said:

That is a great idea!

Unfortunately for me, I don't think it will solve my problem because mine seems to start pumping fine, and I verify water is coming out of the exhaust, and then later while running, the impeller shreds.

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It has to be the pump unless you are buying cardboard impellors direct from China(!)

The wet side of the pump stays cool to the touch, but I'm going to watch the bearing side for overheating.

There could be some sort of debris in the heat exchanger that intermittently blocks the flow (although I have back-flushed it).

There could be some delamination in the exhaust hose, much of which travels through the cabin wall/ceiling and is not visible.

I am starting with the pump because I can bring it home and put it on my bench to work on.

I am going to run water from the dock hose through the system in the exhaust direction to see if that detects a blockage as far as the muffler. I have avoided that test because it could back feed into the engine and I have to figure out a way around that, and it is going to spew water all over that part of the engine room and I am going to get wet, and it is chilly here this time of year.

The last and most difficult troubleshooting solution is replacing the exhaust hose that travels through the engine room and aft salon ceiling/wall into the lazerette and then down to the water line exit. That looks like it will be a real wrestling match, and expensive. So it is last on the list.

Somewhere in that mix is a conversion to an electric pump.

I am also hoping to compare my pump to another identical one that I bought on eBay, but I bought that pump about a week ago and it still hasn't shown up, and the seller isn't answering my inquiries and never sent me the tracking/shipping info. So I am beginning to think that option is disappearing.

I've burned about 5 or 6 impellers in the last 4 months and they are getting hard to find. So even if I figure this out, but later can't get replacement impellers for maintenance getting this pump to work is pointless. I suspect the best solution is the electric conversion. But after all this effort I would like to solve this puzzle.

This morning I reinstalled the pump with the latest impeller.

This is a link to a brief video of the exhaust. Does it look about right to you guys?

http://frankschaffer.net/flow.MOV

This is my first boat, so I don’t have anything to compare it to, and it was raining today so there wasn’t anyone around to ask.

I’ll post a photo too because I’m not sure if I can post a video on this forum.



This is the general arrangement of the generator. The exhaust exits the generator on the left side of the photo and goes into a muffler and then up an out the top left.



I took temperature readings of the pump and the exhaust system after 20 minutes of running.

The impeller chamber read 61.2 F but I think it was picking up some background heat from the cylinder head because it was very cool to the touch.



The pump casing near the pulley bearing was 60.8 F and I think this is corresponds well with what if felt to the touch.



The heat exchanger showed 152.3 F and that seemed about right too.

Here is a link to rebuilding your tapered cone sea cock:https://www.pbase.com/mainecruising/tapered_cone_seacocks
Please keep us posted on your generator water pump.

That's pumping a lot of water.