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2012 yamaha 300 overheating NEED HELP!!!

I have 2012 Yamaha 300hp 30" outboard, the motor is not pissing at all when in the water. it will overheat after less than 10 min at idle. i have taken the lower unit off, the impeller and water pump are both in great shape. I also hooked a air hose up to the pee hole and blasted some air through it hoping to break up any clogs with no luck. when we hook a hose up to the bypass on the side of the motor it will barely ****, when i say barely i mean barely, its such a weak stream it will run down the side of the motor. hoping someone can help me or will know someone who will.


  • Soda PopinskiSoda Popinski GrovelandPosts: 12,612 AG
    Start with the thermostat and poppet valve.   There is definitely a clog somewhere.  after the thermostat and poopet valve, next would be removing the water jacket.  
    You don't need fun to have alcohol
  • alaprovaalaprova Posts: 81 Deckhand
    Thanks we will check the thermostat hopefully that fixes it, if not probably gonna take it to a mechanic and get charged a arm and a leg.
  • Soda PopinskiSoda Popinski GrovelandPosts: 12,612 AG
    Yeah, i always call that plan $
    You don't need fun to have alcohol
  • lukkyracerlukkyracer Posts: 745 Officer
    If it's not pissin your impeller must be bad regardless of how it looks. Jmo
  • TailSpotterTailSpotter St.PetePosts: 44 Deckhand
    Impeller is in backwards???
  • BottomFedBottomFed Posts: 29 Greenhorn

    Yeah, going with lukkyracer on this.  The pee hole is not dependent on the thermostat.  It's direct pressure from the water jacket.  Quick fix is removing the water jacket since that's most likely where the restriction is.  Longer fix is using the bypass or dropping the LU and putting a garden hose over the upper water tube and let it run over night to remove the sand/salt sediment.

    The water jacket is low pressure so you can probably re-use the gasket or, if it breaks, remove it and use Permatex Blue or Red to reseal.  I like to coat new gasket surfaces with Teflon grease to keep the gasket from sticking during future disassembly. 

  • BottomFedBottomFed Posts: 29 Greenhorn

    I shoulda pointed out that since a bypass was used with hardly any flow from the pee hole that this means the restriction is in the water jacket. 

  • alaprovaalaprova Posts: 81 Deckhand
    Appreciate all the help will most likely have a mechanic pull the water jacket, i think its just a little out passed my range of skill and comfort.
  • TarponatorTarponator Under a BridgePosts: 17,001 AG
    edited May 2018 #10
    I had a smiliar problem many moons ago, it ended up being a small piece of shell inside the engine.  My mechanic told me to run a piece of heavy mono leader (80lb) up the pee hole with the engine on and fish it around a bit.  The shell ended up coming out and the engine worked just fine afterwards.  Worth a try before you tear the engine up.
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