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Mercury 4 Stroke 150

Overheating problem

General boating discussion (boats, motors, marine electronics & accessories)

Overheating problem

Postby lsufansr08 » Mon May 14, 2012 8:13 pm

Hey guys, I have a 40 hp merc. Whenever I tried to fish the other day the motor became extremly hot and the water coming out the pisser was almost steam. What would make it get this hot. The water pump seems to be fine and like i said the motor is still pissing. Has anyone had this issue.
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Re: Overheating problem

Postby Ray » Mon May 14, 2012 8:26 pm

Check your thermostat. Might be plugged or stuck.

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Re: Overheating problem

Postby gerald » Mon May 14, 2012 11:03 pm

Pull your thermostate. It is located just above the top spark plug. Be careful when taking the cover plate off. There is a spring behind the plate, so back all 4 bolts evenly until the cover comes off.

Here are a few picture.....I have a 40 hp Force engine.

If your thermostat is dirty, after you wash it off, put it in a pot of water on the stove and bring it up to a boil. When you start to see small bubbles on the bottom of the pot [ie. ~190 F], the thermostate should start to open. When the thermostate opens, it will free up more "trash" and clean it much better.

If it does not open.....buy a new one, for ~ $ 25. I have used the same thermostate for 12+ years and have pulled it many times to clean it and put it back in. Image Image
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Re: Overheating problem

Postby gerald » Mon May 14, 2012 11:11 pm

Another thing.....did you high temperature alarm go off to let you know the engine was getting hot?

Look at the engine head gasket and the side cover plate gaskets. Look to see if there is any "white" color buildup where the gasket is around the edges. If there is a buildup there this means the gasket is leaking [maybe from getting over heated] and should be replaced before it gets to leaking worse.
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Re: Overheating problem

Postby lsufansr08 » Wed May 23, 2012 11:14 am

thanks for the info. Ive been busy and unable to look at my motor, i looked last night and pulled the little cover just like in the picture and the was nothing in there just a hole in the case around the piston and wat not. i looks to me like the stat has been removed? would the motor still cool properly? its has been running fine since i bought it a few years ago. again thanks for the info
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Re: Overheating problem

Postby WestCoveWarrior » Thu May 24, 2012 12:12 pm

No thermostat shouldn't be a problem. Lots of people remove their thermostat on smaller motors that they use for running in the marsh and stuff because of all the trash they pick up. I took the thermostat out of my tohatsu 40 and it runs perfect. I think the thermostat is more for keeping the engine warm in the extreme cold which we don't see down here anyways. My buddy has a 40 merc that was doing the same thing and it turned out he just had his motor sitting up to high and it was turning some serious rpm's, never stopped pissing but would over heat after about 5 minutes of running wide open. He dropped it down a couple notches and it never happened again.
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