This is kinda goofy, but I lose heat (usually on the highway) until I slow down from 60 - 40 mph. But last night I lost heat going 35, until I slowed down to 25. This is my WVO burning 1984 300D. It's gotta be vacuum related, but the car shuts down fine, shifts hard when accelerating hard, shifts softly when accelerating slowly. I know I've PO'd a few people following me, but hey I need heat too,,,

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I had the same problem, its called the monovalve its about 45$ and takes about 10 min to install, on your firewall behind your engine, look for where your heater hoses meet up it will have a wire with a clip and a plate on the top of it. unscrew the top plate and remove the old one, put the new one in and reinstall plate. my heater works great now.

Yeah I kinda was thinking that something is screwy with the mono-valve, as last year I just by-passed it all winter and had great heat, then I had to disconnect it for the summer A/C season.

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That is typical of a torn monovalve diaphragm. Chances are outstanding that a monovale repair kit will solve your problem.