But I wouldn't discount the thermostat, either. I've had a couple stop working over the past decade in my BMWs. I suspect they get old, and I've been told that you can "cook" them--say if you nearly overheat the car for other reasons (like losing a fanbelt). I try to remember to replace them every coolant change.

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TomO
94 E320 Wagon, 170k, totalled in 2006
2002 BMW 525i Wagon, 75k
1991 BMW 318is (two of 'em)