I still remember the SD out in Cincinnati when the temperature was 11°F. Running around town, the temp would just climb over the 80°C. mark. I had to stop at a drug store, so, I figured I would leave it running. Lock the doors with the valet key.

Get back to the vehicle 10 minutes later and the temp. is down to about 73°C. The diesel loses enough heat in conduction to the very cold air that it cannot maintain its thermostat setting.