# Yamaha Overheating



## mgcolli (Jun 25, 2017)

I have a 2001 C40 hp 2-stroke yamaha that seems to be overheating. Sometimes the overheat alarm goes off and sometimes it doesn't. The tell-tale hole is peeing strong and I have already replaced the impeller. I have also replaced the thermostat and tested to make sure it is working (dropping it in hot water). The water coming out of the tell-tale is 140 degrees. Which seems too hot to me, but I am not sure what temperature it is supposed to be. I have also run the motor without the thermostat and it does the same thing. Any suggestions would be appreciated.


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## Dadvocate (Jun 26, 2011)

I'm just putting it out there not a pro. I think that's about the temp the thermostat opens. Did you replace the pump housing also? My guess, build up of deposits blocking water jackets around head. My 2006 25hp was starting to have some when I pulled the head last year to replace a stuck anode. Maybe JC will give a professional opinion.


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## NativeBone (Aug 16, 2017)

Putting aside the temp for a moment, using a thermal reader you can check the temp of the top cylinder and compare to the bottom. Should be within reason to one another. If not, this could tell you if the water jacket is restricted as a result of salt buildup.




mgcolli said:


> I have a 2001 C40 hp 2-stroke yamaha that seems to be overheating. Sometimes the overheat alarm goes off and sometimes it doesn't. The tell-tale hole is peeing strong and I have already replaced the impeller. I have also replaced the thermostat and tested to make sure it is working (dropping it in hot water). The water coming out of the tell-tale is 140 degrees. Which seems too hot to me, but I am not sure what temperature it is supposed to be. I have also run the motor without the thermostat and it does the same thing. Any suggestions would be appreciated.


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## JC Designs (Apr 5, 2020)

NativeBone said:


> Putting aside the temp for a moment, using a thermal reader you can check the temp of the top cylinder and compare to the bottom. Should be within reason to one another. If not, this could tell you if the water jacket is restricted as a result of salt buildup.


This!


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## JC Designs (Apr 5, 2020)

Also, the thermal switch in the head could be failing, but don’t count on that until you are sure! If it were mine, I would remove the T-Stat and pour a 50/50 mix of hydrochloric acid and distilled water down in to the water jackets and flush with lots of fresh water within a couple minutes followed by baking soda and water. Do this with the gear case removed to allow the crud to come completely out. Then, I would make a simple adaptor and use hose to push water the normal direction up the water tube and through the motor pushing the rest out. Might have to do this a couple times and don’t use straight acid.
Or, you can buy a head gasket, pull the head and manually clean it out.


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## NativeBone (Aug 16, 2017)

Damn...its like reading a JC service manual up in here



JC Designs said:


> Also, the thermal switch in the head could be failing, but don’t count on that until you are sure! If it were mine, I would remove the T-Stat and pour a 50/50 mix of hydrochloric acid and distilled water down in to the water jackets and flush with lots of fresh water within a couple minutes followed by baking soda and water. Do this with the gear case removed to allow the crud to come completely out. Then, I would make a simple adaptor and use hose to push water the normal direction up the water tube and through the motor pushing the rest out. Might have to do this a couple times and don’t use straight acid.
> Or, you can buy a head gasket, pull the head and manually clean it out.


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## mgcolli (Jun 25, 2017)

I stuck a hose in the thermostat hole and blasted water at fairly high pressure down it. I then ran it in a barrel and pointed the thermal reader at the top and bottom cylinder. I got 135 on both. Water coming out of the tell-tale was 105 now. Seems more reasonable. I don't know if what I did knocked something loose or this is another instance of sometimes it overheats and sometimes it doesn't. I am still going to do what @JC Designs suggested. I bought the hydrochloric acid (muriatic acid from home depot) today and might try and do that later today or tomorrow. One thing I am worried about is if I pull the gearcase off I'm afraid I will tear the gasket on it and I'll be out of commission until I can get a new one in.


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## NativeBone (Aug 16, 2017)

Before you go down that road, run it a few times with muffs or in a can to determine if indeed an intermediate issue or not. I mean perhaps you did knock something loose and the problem is solved. 



mgcolli said:


> I stuck a hose in the thermostat hole and blasted water at fairly high pressure down it. I then ran it in a barrel and pointed the thermal reader at the top and bottom cylinder. I got 135 on both. Water coming out of the tell-tale was 105 now. Seems more reasonable. I don't know if what I did knocked something loose or this is another instance of sometimes it overheats and sometimes it doesn't. I am still going to do what @JC Designs suggested. I bought the hydrochloric acid (muriatic acid from home depot) today and might try and do that later today or tomorrow. One thing I am worried about is if I pull the gearcase off I'm afraid I will tear the gasket on it and I'll be out of commission until I can get a new one in.


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