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Old 02-09-2019, 05:09 PM   #21
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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.
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Old 02-09-2019, 07:34 PM   #22
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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
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Old 02-09-2019, 07:53 PM   #23
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Originally Posted by Ski in NC View Post
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.
I've been getting them from random vendors, whoever had them in stock, but they are old.

Good tip on the gaskets! Thanks.
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Old 02-09-2019, 07:54 PM   #24
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Quote:
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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
Would happen to remember the flow capacity for your electric pump?
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Old 02-09-2019, 08:20 PM   #25
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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.
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Old 02-10-2019, 07:49 AM   #26
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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.
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Old 02-10-2019, 09:33 PM   #27
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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.
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.
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Old 02-10-2019, 09:44 PM   #28
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Could the intake hose from the seacock be collapsing internally and shutting off the water flow?
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Old 02-10-2019, 09:54 PM   #29
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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.
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Old 02-10-2019, 11:40 PM   #30
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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!
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Old 02-11-2019, 08:02 AM   #31
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Quote:
The intake hose isn't collapsing. It is pretty thick walled, and it is visible during operation.
What can happen is that the inner layer collapses and may not be visable from the outside.
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Old 02-11-2019, 09:20 AM   #32
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What can happen is that the inner layer collapses and may not be visable from the outside.
Oh, I wasn't aware of that. I'll replace that hose then. That segment is only a couple of feet long.

Thanks!
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Old 02-11-2019, 10:57 AM   #33
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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.


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Old 02-11-2019, 04:42 PM   #34
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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.
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Old 02-11-2019, 06:43 PM   #35
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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.
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Old 02-11-2019, 08:58 PM   #36
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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.
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Old 02-11-2019, 11:19 PM   #37
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electric pump work around

The electric pump I used to bypass the rawwater pump was steel, pretty robust. Little giant, 300 gph, 120v
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Old 02-11-2019, 11:42 PM   #38
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The electric pump I used to bypass the rawwater pump was steel, pretty robust. Little giant, 300 gph, 120v
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.
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Old 02-12-2019, 06:45 AM   #39
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"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..
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Old 02-12-2019, 09:46 AM   #40
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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.
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