[E39] Thermostat - how low is too low?

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
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That seems too low 172f. Actually, my understanding is that it should actually run a little higher on the highway to give better economy. In case you didn't know, the thermostat on modern BMWs is computer- controlled for reasons of efficiency and emissions.
One of two things: the thermostat is broken and can't adjust. However, the ECU stores codes in that case (assuming you have them read by a BMW shop that can read the extended codes not just the OBD II codes.)
Or, the fan's clutch is broken and the fan spins at full speed.
I had the 'stat fail on my '01 330xi. Took the opportunity to replace the water pump, hoses, radiator and expansion tank. But I have 110K on mine.
FloydR
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78C is way too low. Two possibilities, the thermostat needs replacement (its possible it's stuck open) or its electrical control is wacko (and I suspect only a thorough dump of the car's codes will indicate that's the answer ... probably something done at a shop specializing in BMW vice Autozone).
OBTW, Mike Miller of BMWCCA and "Bimmer" tech columns thinks you ought to replace the water pump and thermostat as a defensive measure at 60K and the radiator at 90K.
R / John

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"John Carrier" wrote

I had the codes read by a specilized BMW shop. They use the same equipment as a dealer to pull codes. They found nothing.
Pete
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"John Carrier" wrote

Had the thermostat replaced. Still the same issue with temperatures though... if I go 90 mph, my coolant temp drops to 80C. :(
Pete
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Perhaps it was replaced with an 80 degree thermostat??
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"Bob Smitter" wrote

Well, around town it stays betw. 90-95C. Only at higher speeds it drops.
Pete
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operating temperature which could be HOT or COLD depending on where you are.
Like Alaska in the winter the thing would be working hard to keep the engine warm to 90+C whereas in North Africa it would be working overtime to cool the engine to 90-C. Of course this depends whether or not the actual radiator will cope with such extremes?
Many thermostats start to open @ 85C and are fully open around 98C presuming you have a fully pressurized system to at least 15lb.
anything lower than 80C is probably too low for the engine's design.
Hugh
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Sir Hugh of Bognor

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