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Old 02-09-2019, 10:00 AM   #1
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f508's Avatar
City: Belmont, California
Country: USA
Vessel Name: MaryAnn
Vessel Model: 48 DeFever
Join Date: Jun 2017
Posts: 111
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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