[Charles]

I left my '85 944 parked for a week, and then drove it about 5 miles (mostly on a small highway). I parked it for about two hours, and drove it back. The whole time, the temperature gauge never went above halfway (normal for this car). A few minutes after I parked the car from the return trip, I went out and noticed that the coolant was running out of the overflow tube on the expansion tank. (I'm fairly certain that it wasn't leaking on the way in; there was no trail leading into the parking space, and I would have stepped in it on the way to the house).

I figured that if the system had enough pressure to blow past the cap, that it wouldn't be a good idea to open the tank, so I just collected as much of the coolant and discarded it. However, I'm stumped as to how an air bubble could have formed in there while driving-- I'd expect that after adding more coolant without bleeding.

Ideas?

I trust the temperature gauge as being roughly accurate, because when I bled the system, the gauge rose normally from cold (I bled it later that evening) to past the middle, where the radiator fans turn on, and then it went back down.

Since then, the coolant level has been dropping slowly, but I can't seem to catch any dripping when I leave the car parked.