Motor overheating?

I have a 1991 60 horse merc that runs great but when I am idling for a long time, such as backtrolling with it, the heat alarm goes off. It spits water good and runs smooth and everything and the only time the alarm goes off is when it is idling for a long time. I can run all over the lake for a long time and it doesn’t go off. What could be the problem? Could it just be a sensor?

Thanks for any help

I have a ’99 50hp Merc 4stroke.It did the same thing. Alarm at idle, no alarm at higher rpm.
I replaced the impeller, and have not had an alarm sense.

I would change the water pump impellar they are a maint. item anyways. It could have low flow at idle when worn but plenty of flow for the little port that lets you know the pump is working. At high rpm it is probably pumping enough to keep it cool.

Pig-hunter — I had this same problem with my 60 merc. I bought it when it was a few years old. If the water stream is strong at idle chances are its not the impeller. These motors don’t idle down enough to slow troll so a lot of them had the timing [censored] to slow them down. I took mine to a dealer to have it checked– it was 16 degree off at idle. He put it where its supposed to be and it has’nt been a problem since. Just something you might want to check. Hope it helps. Good Luck

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