E39 Overheated briefly, help..

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??
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Check for a coolant leak, pinhole sized. The coolant level is low, but not enough to get hot real fast. I had the same problem and at idle for more than 2 minutes would hit the red zone. There was a big air bubble in the system, 1 liter big. Luckily I caught it in time.
Bill Mississippi

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My V8 did that first time i used the heater in anger this autumn. I rebled the cooling system and it's been fine since.
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Likely there were bubbles/air in the cooling system that eventually (especially after using the heater) escaped the motor (to the reservoir).
If the car overheated with the a/c on, I'd be sure to check the electric fan and it's relay. Certainly a system pressure test to detect a leak is in order, too.
FloydR
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I think you may have a bad gauge. My e30 temperatue gauge developed a similar behavior some time ago. I went nuts trying to figure it out for a while but finally solved the problem by replacing the gauge with one out of a cluster that I got on Ebay. The symptom that convinced me that it was the gauge was that I discovered that sometimes when it was in the red I could get it back into the normal range by giving the instrument cluster a sharp rap with my knuckle. I have heard of other people repairing their instrument clusters by resoldering the connections on the printed circuit board.

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Your thermostat is likely giving up. You might need to replace also the aux fan clutch and maybe the water pump.
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Your thermostat is likely giving up. You might need to replace also the aux fan clutch and maybe the water pump.
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