Dumb question: When was the last time the coolant was changed? I did a flush & refill with a 50/50 mix of MB coolant and distilled water and my S320 was much happier afterwards. I just got back from Orlando, FL (took the Auto Train!) and the car stayed between 80*c and 90*C the entire time we were there. Even in stop & go traffic with the AC turned on.

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"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles