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I have a 1990 Yamaha 90E on my 1990 Montauk 17. Two trips recently when running at 3-4200 rpm in choppy conditions the temp alarm has come on despite visual confirmation of good water flow through the engine - steady stream coming out without stopping when the alarm went on. It seems like this has happened when pounding in the chop. When I throttle back and shut down/restart right away the alarm stops and all is fine until the next hard impact. Does this sound like a loose wire or a sticking thermostat? I do believe I sucked up a little mud earlier in the day this first happened but I was definitely getting good flow after revving up. Any thoughts on the problem or how best to diagnose it?

Thanks in advance!

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When it sounds, what is the temperature of the water? Feel with your hand.

You want to flush the engine alot if you sucked up mud.

How old is the impeller in the lower unit?

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Clean out the thermostat housing, and replace thermostat. Also replace impeller and probably water pump housing and you should be fine. Engines can overheat under load even with a good telltale.

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The impeller, housing, and thermostat have maybe 20 hours on them since replacement earlier this year. I am going to replace them anyway at this point, better to play it safe after getting in the mud. If it truly was overheating then wouldn't the alarm remain active when I start up again (whole cycle - hear alarm to off to engine back running w/o alarm <1 min)? That's what I remember from the one time I did have impeller failure and lost flow and definitely overheated.

Thanks.

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If you restart and alarm does not sound, you need to check the circuits and parts.

I would not replace anything till it happens again and you can reproduce it on demand.

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Had a similar issue on an earlier montauk with a Johnson. Check the wires on the temp sensor to make sure they are not compromised. I actually had to replace the sensor itself. Took some effort to rule out overheat by running without cowling until alarm sounded, then used instant read digital thermometer on the head measure temp. Sounds like a loose wire but you have to methodically rule out all possibilities until you find it

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1979 Montauk with 90 hp 4 stroke Yamaha.

When I had this motor installed, I also had the dealer install a water pressure gauge. Now I do not just use the telltale but check the water pressure also. Not hard to install, but I monitor the pressure as well.

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Thanks everyone. I'll be looking at the sensor wiring asap. Hopefully it's an easy fix. Time to break out the manual before I start. Once I diagnose it I'll update what it was.