I tend to agree with the theory that it is a bad gage.

I don't want to lead you off in the wrong direction, but I did have a "similar" situation with my '92 500 SL some time ago. It seemed to run OK and stay around 90, but hard running in heat on the freeway would cause it to rise up to 110. If I let my foot off the gas abd ciasted at speed, it would come back down pretty fast. I wouldn't do it instantly, but quickly. I found out that I had a couple bad rotor caps that would start arking in heat under load. New caps, rotors and a tune up and all returned to normal. Don't go spending the money for caps, rotors and a tune up until you check the gage and sensing system.

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roadkingmike@aol.com
Coast Aero Support Services, LLC
'92 MB 500 SL
'01 BMW 525i Sport
'98 Harley Davidson RoadKing
'74 BMW 3.0 CSi - Gone to a good home