I posted a few weeks ago about a climate control problem with my 85 300td. Heat would be fine for about a half hour, then cold air would blow. Sometimes I could push different buttons and get heat for a while again, but it would always start blowing cold eventually. I disconnected the monovalve as a result of the advice from that thread and thought the problem was solved. Monovalve open all the time, hot coolant thru the core, lots of heat. Worked fine for the last couple of weeks, except it got too hot sometimes. I was waiting to pick up a new control unit at the Pick and Pull.
Today on the way to work after the car was running for about half an hour, it went cold again. What could be causing the air to cool if the monovalve is not getting voltage so theoretically ought to be letting hot coolant flow freely thru the core?? The engine temperature did not change on the gauge. Is there a flap of some kind that might be allowing cold air to enter the system, overwhelming the hot air from the heater core? I can't imagine that the monvalve is closing since it has no power at all anymore.

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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