Probably bad vacuum pod. It's a tough one to replace. Many people remove the dash to replace it. My solution was to stick a piece of flat aluminum stock down into the vent and bent over the edge of the vent to hold it in place. This kept the flap open. I now control the center vents with the on/off spinner at the outlets. I think I plugged the vacuum line going to the pod also.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13