Anyone have any ideas. On an E39 520, I pulled over recently after
having driven about 45mins. I left the engine running with the AC on,
while I was waiting for about 30mins. I suddenly noticed that the car
temp guage had gone into the red. I immediately switched off the
engine.
I left the car for about an hour to cool then came back and checked the
coolant level and it seemed fine and I also saw the fan turning when I
restarted the engine then,which left me puzzled.

I decided to limp back home, but as I was driving back, the temp guage
never hit red again but practically teased me all the way back by
moving between normal (middle) to abnormal (~3/4 but not red though),
the thing was it was doing it in sudden spikes. First it would drop to
normal, then a while of driving, it would after a moment suddenly spike
to 3/4 of the guage but not red.

What I found confusing is that after a few hours later, it simply
returned to normal when I drove the car and now just seems to be at
normal temp even though I have used the car for long 40mile journey,
the guage now seems to only go to the normal position. Although I
haven't used the AC then and I'm using the heater anyways.

Anyone know what the problem could be as I'm quite confused. I don't
think a car overheats while idling and fixes itself, even though I know
that the E39 are prone to intermittent faults which come and go as they
please. Any ideas??