Seems to be a characteristic of modern diesel motors which are very efficient and produce little heat as a waste by product. I do notice that If I start the van in winter above -5c in the mountains and go on a long downhill road the motor won't reach operating temperature. Fortunately I have the programable upgrade to the coolant heater, so I can turn it on manually and get the motor up to temperature in a few minutes, even if I'm coasting downhill. To be honest, I use it regularly to get the motor up to temp fast from cold starts anywhere, to minimize engine wear when cold. Below -5 the standard coolant heater should kick in automatically, but I agree that there is a grey area between very cold and warm weather where the engine can have difficulty reaching temperature depending on the driving conditions. What you should always avoid is starting the motor and letting it idle to warm up. Most engine wear occurs when the motor is cold and the internal parts have not expanded to fit properly. That is why most manufacturers recommend starting the motor and driving off GENTLY immediately, to get the motor up to operating temperature as quickly as possible.