Super-low coolant temp after driving

[Gurlugon]

My automatic '91 525i had a peculiar problem today. I noticed halfway through my 8.5 mile trip to college, my coolant temp was in the high blue mark. It had raised at a normal pace up until then and then stuck there, or so I thought. I turned off the heat, and it didn't do much of anything for the indicator.

On the way home, I had the same problem. I took it extra easy on the engine in case it wasn't just a thermostat error or something like that.

So I pop the hood once I'm back here at the house, touch the motor, and it's cool to the touch. Same for the radiator's tank; I carefully opened it and there was a low hiss, but the coolant was already cool. The bleeder valve seemed fine, I had enough coolant... It's certainly strange. It was just around freezing today, not any colder than the car's already seen so far this season.

What might the culprit be? I would really hate to damage the motor. For what it's worth, the car drove perfectly normal for it being a colder day, it took a while to shift (bad trans) from the cold start and a few minutes of warmup, but after that acceleration and shifting were normal.

[witeshark]

This seems like a thermostat problem, either stuck open or missing.

[mrmojo37]

My 528i temp gauge would stay in the blue while driving and would go to 12 o'clock position when just idling. I changed the thermostat and now it stays at the 12 o'clock position steady after running for a few minutes. Try the thermostat. Remember to bleed the air out of your cooling system after changing the thermostat.

[drz]

Thermostat

[masesE39]

your thrermo stuck open

[da_bull]

and bleed, and bleed, and bleed, and bleed, and bleed, and bleed, etc. etc.