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  • Looking for Thermostat

    I have a 1990 Yamaha 90 HP and the temp light is now going on regularly, want to check the thermostat but cannot find it. Is there a good schematic out there that can show me, or a pic if available. thanks.

  • #2
    Remove upper cowl, and looking at the back of the motor the thermostat is at the top/back of cylinder head under a square cover with 4-10MM hex bolts....

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    • #3
      Remove upper cowl, and looking at the back of the motor the thermostat is at the top/back of cylinder head under a square cover with 4-10MM hex bolts....Go easy on those bolts so as not to break them off if the threads may have corrosion on them....you may probably need a new gasket for that cover...

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      • #4
        Thanks Robert, very helpful, the bolts came out really easy. I put the thermo in boiling water and it did not change, so I assume it is stuck? Going to contact my local dealer for new thermostat and gasket. Any other advice/suggestions are welcomed.

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        • #5
          There should be movement of the thermostat when put in boiling water...so it's either stuck or faulty. There may also be a pressure control valve/spring/grommet under that cover...but I'm not certain your model has those pieces....if so, then just clean and replace them

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          • #6
            Thanks again, I ordered the gasket and thermostat, let's see what happens when it comes in and install it

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            • #7
              So I installed a new thermostat and gasket and it ran without the alarm for a good long ride, next day the alarm came right back on. For some reason I think it may have something to do with the trim of the engine, possible air to water intake - although water comes out of the pisser hose just fine.

              What should I try next?

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              • #8
                I would inspect/possibly replace your water pump....

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                • #9
                  Ok anything in particular, possibly a link to a diagram or video, thanks

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