[scissorhands]

Your right in that the engine survives these fluctuations, everyday, with no problems. An engine draws cool water from the bottom of the radiator via the impeller wheel on the water pump and the thermostat is like a tap controlled by temperature. The thermostat is located at the point where water exits the engine back to the radiator, normally at the highest position on the head. I have descended hills in neutral with engine on for years in my auto transmission Toyota diesel van. After descent is over and I engage drive I notice the water temp dives quickly. The sensor is mounted in the lower portion of radiator. HOW ABOUT a thin film wind cutting curtain that closes the front grill to maintain coolant temperature would not only conserve warmth in the heated engine etc, but also improve aerodynamics of the car (both increasing mpg and coast time/distance). It closure and opening could be temperature controlled or manually done with a piece of string. Moving off when cold one could keep the engine bay shut so no windage would work to cool engine until ideal operating temp was reached. In cooler or wintery conditions in could be partially or fully shut at times and would minimise heat loss and improve engine performance, mpg, drag