Anyone know the proper operating temp range for a thermostat on an e39 (530i)? Mine will get up to about 92C, but then occasionally drops down to as low as 78C, for example when driving on the highway at 85mph. Is this too low and is indicative of a thermostat stuck open?
BTW, these are the readings from the secret test codes accessed through the instrument cluster. The analog temp gauge itself stays in the same position (straight up) regardless of the actual temp, but that is just because it has quite a wide dead range. However, is it possible that it's some coolant sensor that's defective rather than the thermostat itself?
The reason I even consider this is because despite having a 30 minute commute and living in warm climate (Florida), I still get excessive water concentration in the oil (based on my recent used oil analysis). So, either my engine is not operating hot enough to burn off water vapor or my CCV system is partially clogged. Or maybe you have other suggestions? There are no error codes stored.
FYI, my fuel economy has been down by a few mpg lately, mainly on the highway. The car is an '02 and has 42k miles right now.
Thanks, Pete