My 2555HST service manual states that the thermostat is designed to open up at 72C, about 160F. It also states that overcooling can be caused by ' Excessive low atmospheric temperature ', and can be cured by ' Decrease radiator working area by radiator masking '. As a side note, you have to remember that the heater core in the roof of the cab is pretty large and acts as a secondary radiator [ or heat sink ]. Especially if the two blower motors are running, that's probably more air movement over the heater core than your average vehicle. I am not saying that is the cause, you just have to remember that thing is taking heat away from the motor, too. The heat that a motor makes is actually waste heat. And, though the thing usually runs better at a certain temp, the motor making too much heat is not good either. Again, not condoning the heat problem... Someday, I am going to have enough snow to get mine out and work it.