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Thermostat question

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  • Thermostat question

    I have a 2003 yamaha f115 4 stroke. My thermostat opens at just under 120 degrees f. Once I take it out of the hot water, it takes 2 minutes to close. Seems like this would make it stay open almost all the time in my engine.
    Should I swap it with a new one? I had another New thermostat I bought by accident, different size, but I heated them up side by side and the other thermostat closed in under 15 seconds out of the water.
    I was making oil is the reason I checked it.

  • #2
    you could monitor the water temp as you add some cooler water to see what temp it closes back off,
    I would think it should close back at 115 or so


    • #3
      Two minutes seems to be a long time to close once there's no hot water.

      Possibly, that T-stat is 17 years old.

      If your "making oil" and it takes that long to close, it's cheap enough (much cheaper than an oil change/filter), I'd just replace it...

      How corroded does it look, maybe post a pic?

      1997 Angler 204, Center Console powered by a 2006 Yamaha F150TXR


      • #4
        The thermostat is a brand new one from this Summer. I changed it because the other one had been in there for 17 years, but working fine I bought a yamaha tstat from the local marina. It has an updated number so I think it’s an 01-00.
        I pulled into the dock today after running the boat a total of 10 minutes. I popped the top off to pull it out to bring it home and test it, and it was wide open. I pulled it and watched for a couple minutes as it lay in my toolbox, and finally started closing. If it opens at 118-120, I can’t imagine the motor ever is much cooler than that, so it must be like it's running stuck open.


        • #5


          • #6
            Look at this video, especially at the 11 minute 30 sec mark:

            The T stats are closing in about a minute (yes this is an automotive stat). .

            It is possible the new t stat isn't working properly. There may be a # stamped in the T-stat showing the supposed to be opening.

            Looking at the spec's for my F150, the T-Stat temps ARE a bit higher than yours. The cooler temps, and staying open
            MUCH longer ISN'T HELPING with making oil.

            Also, per my F150 manual (your may be different:

            1997 Angler 204, Center Console powered by a 2006 Yamaha F150TXR


            • #7
              Thank you. I watched the video, and mine is closing much slower. My tstat is stamped 50 which I assume is Celsius which is 122F. Not sure my thermometer is perfectly accurate but seems to be opening at 118, and fully open at 130. I think I’ll suck it up and buy a new one, then I can compare.


              • #8