My grey car has a new Bosch monovalve and I have never had a problem with it. The silver car got a cheaper MTC monovalve last fall and it worked for a little while but now I have no heat in that car at all. Even if I unplug the monovalve, no heat. I did do some cooling system maintenance a few weeks ago. There may be air in the heater core but after driving on the highway for a few weeks, it should have been pushed out by now.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge