I ran the 1991 40hp evinrude on the muffs Fri. All was well at the start up P stream ok water cool then starting to get warm then very HOT from the P stream so hot it was not comfortable to hold your hand in the stream. I shut the motor down let it set for a while then restarted no P stream I shut the engine down and took the T-stat out replaced the t stat cover started the engine then the P stream returned cool I am assuming the return of the p stream was because the stat was removed I did not let the motor run long enough to see if it got hot again trying to be cautious about this. The Impeller was replaced Early spring and I have had the boat on the lake 2 times since then and all was well. I run this motor once a month on muffs for a short time to keep the impeller in good shape if I can not go to the lake. I tested my T-stat today on the stove in a pan of water. The Stat starts to open at 147 and is full open at 177 is this ok or should it be full open at say 140 or so(should it start opening at a lower temp and be full by 140-143?}getting very hot water from the p stream this has never happened before water pump replaced in Feb 2011
I have tried to find temp range on the internet on the #433379 t-stat but no go.
1991 40 hp evinrude
Tom H

how old is the stat?

about 4 yrs.
It is opening At 147 full open at 177 I just don't know if this is too high to open.
Tom H

The water from the tell tale or "p stream" is going to be hot. Its picking up heat from your motor.....that's the way its designed to work. The water circulates through the motor and picks up heat.

Its also going to be hotter if you start out with 95 degree water instead of 60-70, the hotter it is going in the hotter it is going out. T-stat opens at 177 but that don't mean that the water wont be hotter than that. I think your good as long as psi is up there and you do not have a blockage.

I put a large garbage can under the motor and filled it with water well past the pick up vents. I reinstalled the T-stat started the motor and got a very strong P stream. I used a meat thermometer with a probe to check the p stream temp. all said and done the temp. from the p-stream got to 105 and never higher this time. Only thing I can figure is the T-stat was stuck closed. I have ordered a new one and will install.
Thanks to all for the input.

Tom H.

The thermostat should open at 143degrees and be full open at 143 degrees. The external temp of the block should not reach more than 145 degrees. The t-t stream is and should be about 30 degrees less than the thermostat. With that being said. You shold be able to keep your hand in the pee stream for at least 5 seconds. Also if you remove the thermostat completely, the engine will super heat the pistons and the block will not expand because the water is rushing thu it and not pick up the temp. So the block is cold and the pistons are hot. Pistons expand and block is not,,,,,excessive wear on pistos to cylinder is taking place. It could gauld your pistons or at the least prematurely wear your engine out.