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It is probably a bad vacuum servo that is activated when the heater is off and not activated when the heater is on. The one that I suspect meets this criteria is the one that lets fresh air into the system. I'm pretty sure this is the one behind the glovebox to the right. I would pull out the glovebox and check this one first. It could be as simple as the line being disconnected to that servo.
I dont know if any other vacuum servos meet this criteria.
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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
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