Hi Ian, I am very interested in your above comment as it is completely the opposite to my experience. In a former life I worked in the automotive aftermarket. One of the product groups imported into AU by my employer was engine thermostats manufactured in Germany & Israel from a very reputable manufacturer. One of my responsibilities was to process claims when end users or workshops submitted claims for alleged faulty thermostats. In the over ten years in that role I can not remember one claim for a thermostat that caused the engine to run below operating temperature. Just the opposite, overheating with subsequent damage ( often head gasket issues) was the subject of the claims. While I do not wish to bore you with the details of a typical claim, the end result was almost always that the thermostat was a victim of another failed component in the cooling system. The thermostat is simply a valve that in the closed position it separates the radiator coolant from the coolant in the engine. When the coolant in the engine is heated by the combustion process to the temp setting calibrated into the thermostat wax pellet it overcomes the pressure of the return spring and opens. Generally there is a small temperature variation between the "start to open " temp and "fully open" temp, from memory around 5-6Deg C. I hope the above explains why I made the comments I did. Cheers, Gary