Thermostat Yes or No

[Grumps]

I have a '91 Evinrude 70 hp. While out on the lake Saturday, the overheat buzzer came on twice before I unplugged it. I unplugged it because the tell tale pee stream was cool and plentiful. My question is what are the pros/cons to removing the thermostat? On this motor the thermostat and the overheat sensor are just above the top cylinder. I'm thinking of running a test on the lake without the thermostat to see if the buzzer comes on again.

 

[nccatfisher]

Pros, none.

 

[JL8Jeff]

The overheat warning buzzer went off on my 60/45 Mercury but the water was still cool. I got a new temp sensor and the buzzer still goes off so it needs the module. I just unplugged it and it still runs cool. I don't run more than 10 minutes at a time so for me I don't think it's an issue. If you run longer, I would want to replace whatever is bad so you don't damage the motor. If the thermostat isn't bad, then something else in the overheat circuit is and you probably want to fix it. I'm not even sure if my motor has a thermostat or not, the water coming out the telltale never gets overly warm.

 

[thill]

NEVER unplug the buzzer or overhead sensor without doing a check first.
If your T-stat is stuck closed, the water will bypass the head and be cool, but the motor will still overheat and could be destroyed.

Did you pull the cowl off and put your hand on the motor? Could you hold your hand on it? If the motor was warm, but not scalding hot, it could be a bad sensor or damaged sensor wire grounding out. If the motor was too hot to keep your hand on for more than a second or two, then it was overheating, and the sensor was doing it's job.

Removing the Tstat is a viable short term solution, especially if stuck on the water, but you really want to replace it ASAP. Same thing with the sensor. The cooling and overheat system is a vital part of the engine, and is foolish to disable... if you like your motor, that is.

Hopefully, it was nothing too bad.

 

[bbot1967]

Also need to consider the possibility there's a small head gasket leak causing a bubble inside the water jacket. At low speeds it might not be much of a bubble but higher speeds the bubble will build up. Had this happen to me with a '90 Yamaha 30hp. Had I ignored the thermostat it is likely there would have been major damage. Head gasket isn't as complicated as one would think.....er......at least for me : )

 

[Pappy]

Stop it with the head gasket leak.
In this forum we strive to be as accurate as possible with information. Poor information or incorrect information often causes the engine owner to spend needless money. Furthermore a head gasket leak will show up immediately in the running quality of the engine and spark plug. Spark plug will be very clean in comparison to the other two.

 

[Pappy]

Now that that is out of the way........a thermostat warms your engine at idle and the idle quality is greatly improved as a result. Also it allow the engine to warm sufficiently in colder water to eliminate moisture build up in the cylinders and rust corrosion down the road resulting from that. Your engine does not run on the thermostat all the time. It cannot flow the volume of water necessary to cool the engine at higher RPM and load. The thermostat sits inside a pressure relief valve assembly and the whole assembly is blown off it seat by increased water pressure and volume as throttle is increased and water pressure and volume follow suit. This effectively takes the thermostat out of the cooling circuit. This is why the common practice of diagnosing the condition of the water pump by looking at the pee stream or feeling it's temperature is total bunk. It tells you nothing about how well your pump will supply needed pressure and volume at high RPM and load. Check and replace as necessary!!
Why is your pee stream cool and the engine hot? Because the pee stream is before the thermostat! That is why it appears so quickly in your engine vs. waiting until warm and thermo opening.

 

[Grampy]

Also need to consider the possibility there's a small head gasket leak causing a bubble inside the water jacket. At low speeds it might not be much of a bubble but higher speeds the bubble will build up. Had this happen to me with a '90 Yamaha 30hp. Had I ignored the thermostat it is likely there would have been major damage. Head gasket isn't as complicated as one would think.....er......at least for me : )

 

[nowgrn4]

Now that that is out of the way........a thermostat warms your engine at idle and the idle quality is greatly improved as a result. Also it allow the engine to warm sufficiently in colder water to eliminate moisture build up in the cylinders and rust corrosion down the road resulting from that. Your engine does not run on the thermostat all the time. It cannot flow the volume of water necessary to cool the engine at higher RPM and load. The thermostat sits inside a pressure relief valve assembly and the whole assembly is blown off it seat by increased water pressure and volume as throttle is increased and water pressure and volume follow suit. This effectively takes the thermostat out of the cooling circuit. This is why the common practice of diagnosing the condition of the water pump by looking at the pee stream or feeling it's temperature is total bunk. It tells you nothing about how well your pump will supply needed pressure and volume at high RPM and load. Check and replace as necessary!!
Why is your pee stream cool and the engine hot? Because the pee stream is before the thermostat! That is why it appears so quickly in your engine vs. waiting until warm and thermo opening.

Deflector issue in the head water jacket in a 31 year old engine?

 

[Pappy]

Deflector issue in the head water jacket in a 31 year old engine?

????? this is a lost foam casting process in this engine. You are thinking of a cross-flow V4 or V6.

 

[Grumps]

Now that that is out of the way........a thermostat warms your engine at idle and the idle quality is greatly improved as a result. Also it allow the engine to warm sufficiently in colder water to eliminate moisture build up in the cylinders and rust corrosion down the road resulting from that. Your engine does not run on the thermostat all the time. It cannot flow the volume of water necessary to cool the engine at higher RPM and load. The thermostat sits inside a pressure relief valve assembly and the whole assembly is blown off it seat by increased water pressure and volume as throttle is increased and water pressure and volume follow suit. This effectively takes the thermostat out of the cooling circuit. This is why the common practice of diagnosing the condition of the water pump by looking at the pee stream or feeling it's temperature is total bunk. It tells you nothing about how well your pump will supply needed pressure and volume at high RPM and load. Check and replace as necessary!!
Why is your pee stream cool and the engine hot? Because the pee stream is before the thermostat! That is why it appears so quickly in your engine vs. waiting until warm and thermo opening.

Thanks. One of the best answers I've had about this issue. Just started a new thread about relocating the pee stream to the top of the motor as outlined in the '94 service bulletin. Need to look at my thermostat first just incase it is bad......or stuck.

 

[Pappy]

If you have not relocated the hose for the overboard indicator that is an indication you need the updated waterpump assembly as well. The kit will include a different spring for the pressure relief system I talked about earlier plus other updated parts. Well worth the investment.