The first thing is to find out if the temerature gauge is correct. You can do this by using an infrared heat gun (display) to show the tempaerature and they do it in both
celsius and Fahr. The themostat controls the water flow but if stuck in the OPEN position then the water would flow too quickly through
the radiator and be cooling too much. This scenario does happen but not as often as stuck closed which blows head gaskets. It would be nice if they would be designed to always stay open when defective!
300SE, 300CE, 190/5.0