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onan raw water problem

Discussion in 'Post Yacht' started by bamp, Jul 30, 2017.

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  1. bamp, Jul 30, 2017
    #1
    bamp

    bamp New Member

    Joined:
    May 8, 2012
    Messages:
    23
    Location:
    winter hill yacht club somerville ma.
    hi guys I have a 1980 onan 6.5 generator with a raw water problem. Changed the impellers on the orbendorfer raw water pump because not pumping enough water. Only 2 blades out of 5 but still pumping. Now will not pump unless I remove water line from exhaust elbow for about 5-6 sec with the genny running then the water flows great. When I put it back on exhaust elbow ( with the genny running) the water flows and cools great. It will run all day but if I shut it down and start it back up it will stop pumping until I repeat the process. The exhaust seems to have to much pressure coming out of water fitting on the exhaust elbow. Will check for rotted elbow inside but the strange thing is old impellers with only 2 blades pumped but not great. I did remove the broken blades. The only thing that changed was the new impellers and I got the original repair parts for orbendorfer. I am stumped. Any help greatly appreciated. Thanks in advance.
  2. Pascal, Jul 30, 2017
    #2
    Pascal

    Pascal Senior Member

    Joined:
    Feb 29, 2008
    Messages:
    8,149
    Location:
    Miami, FL
    Sounds to me like the pump is sucking air and not priming unless you help it. Check the seal on the cover or any hose you may have pulled
  3. Capt Ralph, Jul 30, 2017
    #3
    Capt Ralph

    Capt Ralph Senior Member

    Joined:
    Sep 8, 2004
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    12,725
    Location:
    Satsuma, FL
    Yep
  4. Capt J, Jul 30, 2017
    #4
    Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    14,434
    Location:
    Fort Lauderdale
    It sounds to me like the bearing is going bad on the raw water pump and the seal is leaking and it's sucking air. Is the front of the pump all green with salt/corrossion where the shaft comes out of it?
  5. chesapeake46, Jul 31, 2017
    #5
    chesapeake46

    chesapeake46 Senior Member

    Joined:
    Jul 26, 2009
    Messages:
    1,781
    Location:
    Chesapeake Bay, Delaware Bay & S.Jersey
    I have a 7.5 Onan. Mine is set well above the waterline in top of the fuel tank in the bilge.
    It's about 3 feet above the waterline to the pump inlet.
    My pump would loose prime a lot and I was changing impellers a few times a summer.
    I tried a lot of remedies including rebuilds and new pumps but finally put a check valve in the bilge near the seacock and the problem went away. I was surprised that the water pump could overcome the check valve spring but it does and the problem is gone.