A properly operating thermostat should maintain correct operating temperature. However, a diesel engine is SOOOOO efficient, it does not generate a lot of heat.
I used to drive a diesel car which was super-efficent (over 50MPG) In the winter, it would spend the night plugged in to heat the antifreeze. When I started it in the morning, the engine-temperature was measuring on the gauge.
Once I started driving, the engine-temp would go DOWN if I tried to heat the interior of the car. The heater-core would suck all the heat out of the engine.
Bottom line, I cut up a cardboard box to create a cover for the radiator. That cover stayed infront of the radiator until outside temperatures went above 20F.