I has this exact problem on my '86 300E with 170K miles. My tech pulled the dash and replaced some vacuum pods (servo?. It worked great after that. Actually, my problem evolved to the point where there was NO air blowing out of the center vents. Coming from Houston this was totally unbearable!

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Mark Herzig
1992 500E (82,000 miles)
1987 300SDL (105,000)