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Yamaha 70BET0 overheating alarm

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  • Yamaha 70BET0 overheating alarm

    Hi everyone,

    I have an old 70HP 2 stroke (1997), it runs great but on my last 3 trips I had the overheating alarm buzzing.
    I'm trying to better understand the problem (and make sure there is actually an issue). This is what happened in my last 3 trips:
    • 5-10min after launching the boat and running at low speed (8kn zone), the overheating alarm starts and the motor goes in safe mode.
    • I turn off the motor, remove the cowling (the heads is warm but not super hot), wait for a few minutes, start again and the alarm is gone for the whole day (30miles trips)
    • The impeller has been replaced a few months ago
    • The telltale is not super strong but it has always been like that and seems works fine

    If there is the motor overheats, I don't understand why it is happening only once in the first 5-10min. I just have to stop and start again the alarm is gone. Then I can run the motor all day and the problem doesn't come back until next time.
    Now, I live in Australia and it's summer here and when this happened it was pretty warm: air temperature at 90F/30C and water around 75F/24C, I don't know if this could be related...

    If you guys have any idea what's happening, please let me know.


  • #2
    I would pull the stat and check that it operates in the spec range.
    pot of water and a thermometer are needed to see what temps it opens and closes at.

    might be sticking closed but the opens .
    while you have it out check to see how much scale ,salt, or corrosion there is inside the opening where the stat sits.

    also I am not sure it your 70 has a PRV that could be sticking open that would cause a low RPM overheat.
    something to check on if it has one