Note that the coolant level in the recovery tank acts like a thermometer. The level gives a general idea of the ECT. The coolant expands with temperature, just like the fluid in a thermometer. (The engine expands too but not as much.) So if the engine is cooled to an ambient of 80 F and you run it up to operating temperature, 195 F, the
Still curious as to why after driving for more than 30 minutes, the coolant in the overflow tank is still ambient temp. I guess the coolant system anatomy of the duramaxes could be different than gas engines im familiar with and they don't open up to the tank unless things get really warm. My digital gauge says coolant never gets above 183*.