Mercury Marine 25 HP 2-stroke overheat

My Mercury Marine 25 HP 2-stroke Serial Number 0G002xxx got hot and shut down the other night when returning to the boat ramp. I didn’t realize why it **** down until I tried restarting it. It wouldn’t even crank - the starter just stalled.

Thank goodness for trolling motors!

After getting the boat home I tried cranking it a couple of revs and it seemed to crank normally.

The water pump impeller is shredded so that is a fairly easy fix. I decided to remove the thermostat to check it out. I was shocked to find that there wasn’t a thermostat under the cover.

I have two questions:
Should this mid 90’s motor have had a thermostat installed, or did some unscrupulous mechanic leave it out when it was serviced in the past? I am likely the third owner of this motor, so I have no history of repairs.

Should there have been some sort of overtemp switch that would have shut down the motor before reaching such extreme temperature?


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I believe it did have a thermostat. I can't say as to the reason it doesn't have one at this time.

Sounds like the previous owner had heating problems and took the thermostat out. No overtemp switch but you should have an over heating horn that sounded off.

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Originally Posted by Barnacle Bill

Sounds like the previous owner had heating problems and took the thermostat out. No overtemp switch but you should have an over heating horn that sounded off.

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Would the alarm unit be external somewhere near the console or on the motor itself?


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I have seen them mounted near the console or in the controls box if you have one. My 95 Stratos has the controls built into the side of the boat. The buzzer/ horn is right close to them

On this deal, yes it should have a thermostat. First thing you need to do is back flush that thing from thermostat housing back through water tube to make sure no piece of impeller is up in cooling system. If it is hand control and it has buzzer it will be around motor some where. If remote it will be in throttle control.

It would help us do some research if you could give us the year model. I can’t find anything on that number you posted.

I believe it is a 1994, from a bass tracker panfish


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How do I identify my Mercury outboard motor?
The serial number is your key to obtaining accurate parts information and availability for your Mercury Outboard. You can find the engine serial number on the starboard side of the engine on what's called the “transom bracket”. The number will start with a 0 or 1, then a letter (eg. A, C, D, T) then 6 numbers.

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Originally Posted by Cray

It would help us do some research if you could give us the year model. I can’t find anything on that number you posted.

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According to the serial no. he gave it is a 1994.