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3.0L Merc running hot....sometimes

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  • 3.0L Merc running hot....sometimes

    Hey yall, just pu the boat in for the first time this season and went for a warm up run. She ran great for a few minutes then I smelled something, checked my gauges and noticed the engine temp was well above 220. Figured I picked up some weeds on launch threw her in reverse and the temp came back down to 130. After running a cruise speed the temp would creep back up till I stopped and idled or threw it in reverse. After pulling into the dock I could hear water boiling in the block. Disconnected water hose from stat housing to impeller and I could blow thru it and also pulled hose to the water pump and lots of water poured out. If I was run the boat with the supply hose from the impeller off it should be a good stream of water coming out? Im guessing the impeller is shot but i think I will replace it and the stat. She ran good on the trairler with the muffs, plenty of water coming out the exhaust sat right a 130. Any ideas??? Btw it is a 1993 vip 3.0l

  • #2
    Re: 3.0L Merc running hot....sometimes

    Impellers need replacing every other year, period. It sounds like you are in need of one. It is a good idea to replace the thermostat regularly also. Sounds like you are on the right track. When replacing the Thermostat be sure to get the gasket that has the small eyelets in it so your temp gauge will work. Also while you have the drive up or off check the exhaust passage for parts of a flapper that could restrict the flow and also cause overheating.
    When will people learn that impellers require regular
    replacement and are not drive it till it pukes items?

    1984 Regal Medallion 195
    3.8 V-6 with the dreaded
    OMC Stringer Drive.
    2003 Avalanche w/Hypertech

    If it ain't fixable - don't break it!

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    • #3
      Re: 3.0L Merc running hot....sometimes

      Got the new parts but took her out for one more test run last night. Cruising around on plane at no more than 1500rpm temp stays at 130, get going any faster and the temp rises. Drop back down to 1500 r's and shes fine

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      • #4
        Re: 3.0L Merc running hot....sometimes

        Was there any vanes missing from your old impeller? If so, and you didn't find all the pieces, they could be partially blocking a passage. You may also have a faulty thermostat that isn't opening all the way, or even the water passages plugging up in the exhaust elbow and/or manifold.
        Don S.


        Please, no PM's (Private Messages) regarding boat/engine problems.
        That is what the forums are for.
        Only forum/moderator issues will be answered in PM's.

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        • #5
          Re: 3.0L Merc running hot....sometimes

          sounds like a blocked passage way somewhere or a stuck closed thermostat (more likely). if it was a bad impeller, in my opinion you would be overheating at low rpm instead of high rpm when water is forced into the motor.
          2003 Stingray 220LX 5.0L -- Alpha 1 Gen 2
          1977 Masterline 17' -- 165HP, 4.1L -- Alpha 1

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          • #6
            Sign up today
            Re: 3.0L Merc running hot....sometimes

            I installed a new impeller and housing. The impeller didnt look to bad, a couple of wear marks there the housing grates are. The housing on the other hand had a ruff spot on about 1/3 the way around. I figure at low rpm it was pumping good water but a high rpm the ruff spot was creating a cavatation thus letting alot of air in. Just my 2c. Did also put in a new stat for good measure, looked like the factory original. Thanks for the help.

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