[sjgrimsley]

With my car, coolant constantly circulates from radiator to engine block to heater core and back to radiator, no matter if the thermostat is opened or closed. This 406 heats up to operating temp fairly quickly even in cold weather (195° thermostat). When I data log with my laptop it will usually show about 190° unless it's over about 80° ambient temps, then it runs up to 205°. I blame the variation on the simplistic design of the t-stat.

Anyway, before the 406, it had the factory 305, which in the winter time would require blocking about half the radiator with cardboard, otherwise it would never reach operating temp.

I think (I could be mistaken) every engine is designed with a t-stat bypass to maintain a minimum flow of coolant thru the engine to minimize hot spots from forming in the water jacket. I guess sometimes the weather is cold enough the engine can't generate enough heat to soak the cooling system, so it never overcomes the heat loss from that bypass until you come to a stop and let the heat soak happen. Just my estimation.