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Old 22-04-2016, 12:15   #1
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Stumped by waterpump

I just replaced my impeller in my Onan MDKAU genset and I can't get it to pump water. I have removed and cleared all of the lines between my thru hull and the intake to the pump. No issues there. The pump itself is the bronze version of the standard sherwood pump which is plastic and was replaced two years ago. There is no sign of wear on the inside of the pump. This is the 3rd impeller I have installed since I replaced the original pump. I have never had an issue getting it to prime before. I simply replaced the impeller, lubed it well and started the engine. The pump intake is about 3 feet above the water so it has taken 15 seconds or so before I started to see water in the past. This time no water at all. I decided a bucket test was in order, so I ran a hose from a 5 gallon bucket to the intake and it lifts the water a couple of inches in the tube, but not enough obviously. The pump is clearly not producing enough suction to lift the water. If I place the bucket above the genset and have the tube filled when I start it does move water from the bucket. I have replaced the O-ring under the cover and checked it several times and don't believe it's the source of an air leak. There is no water leakage around the shaft, so I don't think the shaft seal is the source of an air leak. Does anyone have any idea? I'm stumped. I have also checked the output side for blockage including removing all of the pieces of the old impeller from the heat exchanger. I have blown the outlet hoses and get no back pressure of any significance. I have also removed the outlet hose completely and there's still no water.
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Old 22-04-2016, 12:42   #2
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Re: Stumped by waterpump

Quote:
lubed it well
Get the vanes in backwards, maybe a ton of lube reduces friction to a point they cant flip over.

Edit: Some big Jabsco bumps have so thigh and long impellers vanes that there would be no room physically for them to flip.
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Old 22-04-2016, 13:30   #3
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Re: Stumped by waterpump

I had the same problem with my Volvo md6a. Changed the impeller and it wouldn't suck water. Finally went and bought another one. Put it in and worked great. Upon closer inspection the first new one was about 2mm thinner than the original and the second new one. Same part number and original Volvo parts. Shorter impeller wouldn't seal against the cover.
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Old 22-04-2016, 13:37   #4
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Re: Stumped by waterpump

Wouldn't be the first time someone got the wrong impeller, either from a wrong parts lookup or a simple inventory mistake. Compare very carefully to the old one. And I'd suspect impellers can be counterfeited (or just made poorly) the same way that thermostats, spark plugs, v-belts, all that kind of stuff is. Globally.
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Old 22-04-2016, 14:35   #5
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Re: Stumped by waterpump

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I had the same problem with my Volvo md6a. Changed the impeller and it wouldn't suck water. Finally went and bought another one. Put it in and worked great. Upon closer inspection the first new one was about 2mm thinner than the original and the second new one. Same part number and original Volvo parts. Shorter impeller wouldn't seal against the cover.
I got out the calipers and measured. It's right on spec. I checked 4 different sources and all of them say this is the correct part. It's a sherwood 8000 which is what was in there before.
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Old 22-04-2016, 14:39   #6
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Re: Stumped by waterpump

Do the bucket test again, but this time, use a greased sheet of glass or lexan as the end cover, so you can SEE whats happening inside.
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Old 22-04-2016, 14:39   #7
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Re: Stumped by waterpump

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Originally Posted by On The Water View Post
Get the vanes in backwards, maybe a ton of lube reduces friction to a point they cant flip over.

Edit: Some big Jabsco bumps have so thigh and long impellers vanes that there would be no room physically for them to flip.
I checked the lay of the vanes and they are correctly oriented. I rotated it backwards while the cover plate was off and the vanes layed over in the other direction without any problem and came back to the correct orientation. That doesn't seem to be a problem.
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Old 22-04-2016, 14:40   #8
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Re: Stumped by waterpump

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Originally Posted by sy_gilana View Post
Do the bucket test again, but this time, use a greased sheet of glass or lexan as the end cover, so you can SEE whats happening inside.

Good idea.
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Old 22-04-2016, 14:44   #9
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Re: Stumped by waterpump

put the old one back in and see what happens
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Old 22-04-2016, 16:05   #10
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Re: Stumped by waterpump

I had this same problem once and it was an air leak in the raw water strainer. The cork had become so thin it was porus (sp?). I made new gaskets and the probkem went away. My strainer was above the water line. A temporary fix was to lower the strainer below the water line. Good luck.
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Old 22-04-2016, 18:30   #11
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Re: Stumped by waterpump

I'm with Charlie. The majority of the (many) times my Onan didn't pump well turned out to be air getting in somewhere on the suction side.
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Old 22-04-2016, 18:42   #12
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Re: Stumped by waterpump

Another thing that will cause that is a worn cover plate. Are there scratch marks on the inside of the cover? On most Jabsco pumps you can just flip the cover over and use it that way. Write the numbers down first as they will be worn off with the cover flipped.
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Old 22-04-2016, 19:07   #13
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Re: Stumped by waterpump

I had a similar issue recently with my M25XPB and a Sherwood G908. In my case the shaft on a replacement 908 was maybe 1-2mm "deeper" in the pump than the older unit. This coupled with a possibly thicker than normal homemade cork gasket caused the shaft to not engage the cam drive of the engine ( I think its a cam drive..). I took the pump apart twice and then took it back to the guy who rebuilt it before I figured it out. I only figured it out when I had the old pump and new pump side by side and it was then obvious that one pump's shaft was sticking a bit further out than the other. The fix for me was to create a thinner gasket out of paper and not cork.

I imagine your pump is totally different and none of this will apply to you. But, maybe this helps someone else down the road.
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Old 23-04-2016, 09:24   #14
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Re: Stumped by waterpump

Sounds like a frustrating experience, Bill.

I agree it sounds like possible air ingress on the suction side [assuming the new impeller is working as expected...]

Looking ahead to future changes that may be of interest...

Two boats ago I started plumbing my raw water wash down pump output [Ts with valves...] into the raw water suction side of all engine coolant loops as a back-up and primer. This has paid off a couple of times in critical situations, but doesn't work as well if bits of impeller block coolant passages.

On our current boat we have a Panda 10kw generator. It has had both the alternator and water pump replaced with external sources. (i.e., no belt...)

The alternator is just an idler pulley in case a belt driven water pump is of interest in the future.

There is no need on this boat for a generator alternator as when there is AC available [or the main engine is running, it is sunny or windy...] all batteries are being charged.

Why no water pump either?

When the manufacturer had a batch of bad impellers years ago- but didn't admit it until 6 months later- the previous owner bypassed the built-in water pump replacing its function with a spare 110VAC March pump [used for the raw water coolant loops for the air conditioning and refrigeration systems.] Ten impeller changes in 2 months would make me re-engineer that system too...

He also relocated the heat exchanger to outside the generator's acoustic housing so it could be replaced with a larger exchanger [and is more readily serviced... They were in the tropics at the time...]

These changes make everything much more serviceable and provide redundancy if one wanted to put a belt back on to run the built-in water pump [with a functional impeller...]

We don't hesitate to properly [and cautiously] re-engineer [optimize] systems using standard components, methods, and procedures, but only when necessary... [Who needs to make more work on a boat?]

I look forward to learning what is keeping your pump from working as expected. And for your sake, I hope that is soon...

Cheers!

Bill
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Old 23-04-2016, 10:23   #15
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Re: Stumped by waterpump

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Good idea.
I once had a jabsco engine pump that would not prime due to OVER lubing the impeller. No suction due to lube, clean and install with a drop of soap and water. Fill hose beyond pump if possible to prime.
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