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  1. #1

    ONAN Raw Water Pump Will Not Prime

    Today's frustration was the water pump on the ONAN 7.5. New impeller, new gasket, seacock open, manually primed with a hose and no water out the discharge side. No obstructions up or down stream. Belt properly tensioned. WTF

    I took the whole pump home, poured a martini and hope some divine intervention (or suggestions from you guys) will lead me in the right direction.

  2. #2

    Re: ONAN Raw Water Pump Will Not Prime

    First......Drink the martini.....then drink one for me.

    Check the cam. Maybe the cam screw came out and it's spinning, which will hose your bore, or maybe the screw came out and the cam fell out into the bilge when you replaced the impeller.

    Check for air holes, like a zinc on the intake side that's loose, a hose connection that's loose, a zinc bulb that's loose, etc. Air will hose your suction in the pump....I guess that's called an "air hose".

    Doug Shuman

  3. #3

    Re: ONAN Raw Water Pump Will Not Prime

    Thanks Doug,

    I poured a Grey Goose- dirty for you.

    There is no cam. It's an offset impeller housing that does the magic. I have a spare spare that I can use to confirm I was given the right impeller. Air on the suction side? Maybe. It's good through the suction hose. Maybe the gasket is bad. Maybe the shaft.

    Hey, if your not going to drink that martini, I'll take it. (Maybe I should hold off serious diagnostics until the morning )
    Last edited by Passages; 04-21-2006 at 08:53 PM.

  4. #4

    Re: ONAN Raw Water Pump Will Not Prime

    Those pumps are self prime. There is not much to go wrong. You must have an air leak or the wrong impeller. In as much as you don't have a cam.


  5. #5

    Re: ONAN Raw Water Pump Will Not Prime

    The seacock is "open" but is the seacock lever actually moving the ball inside it? I haven't been there but friends have. You might want to verify that water is actually coming up the hose. Or that something isn't blocking it. Could the hose be collapsed? I would think that if you had an air leak in under suction you would also have a water leak out when the impeller wasn't turning, but maybe the leak is small.
    These pumps operate on a flooded inlet but they will self-prime, they do generate enough suction for that. Good luck finding it.

  6. #6

    Re: ONAN Raw Water Pump Will Not Prime

    Problems like that can drive one to drink alright.
    What caused you to replace the impeller in the first place? Was the pump working before you tried to replace it?
    How are the splines or the key if it has one, at the impeller and at the drive pulley?
    Last edited by Will; 04-22-2006 at 11:56 AM.

  7. #7

    Re: ONAN Raw Water Pump Will Not Prime

    My generator intake is a round flat plate drilled with many small holes. A few years ago I had the bottom painted. They must have used some sort of power roller to apply the paint because it went on so thickly that it completely clogged most of the holes. After launching it took several days running to chew up the impeller. Other times I have gotten sea grasses in that intake. The inside strainer screen is usually clear--it's the outside part of the strainer that clogs.
    Jim Grove, Fanfare 1966 50MY

    "LIFE IS JUST ONE DAMNED THING AFTER ANOTHER." Frank Ward O'Malley, journalist, playwright 1875-1932

  8. #8

    Re: ONAN Raw Water Pump Will Not Prime

    Try disconnecting the raw water hose just past the strainer or at the raw water pump and see if you have raw water getting to the pump. Hard to imagine a big enough air leak in the hose so it must be no water or a problem with the pump.

  9. #9

    Re: ONAN Raw Water Pump Will Not Prime

    Just a thought ... when you pulled the old impeller ... were all the splines on it? If one broke off it could have blocked the water flow into the genny!

    I helped a friend pull the impellers on his engines last year ... the port engine had three of the 10 splines broken off. We used a shop vac and managed to get all out but one (we pieced the splines back together to make certain that we got them all) ... we finally had to remove the elbow and found the one missing spline!

  10. Re: ONAN Raw Water Pump Will Not Prime

    Yeah, it bites when you pull and impeller and find MISSING splines - that means (1) you have a lot more work ahead of you, and (2) you waited too long to change it
    http://www.denninger.net - Home page with blog links and more
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