Alex,

I don't understand the function of the mono-valve but it does mess up the control of heated air. My car was having irregular heat problems.....sometimes warm, sometimes cold, etc. I found the mono-valve was torn. When fixed, all was well.

Apparently many of our older Benzs have a bad mono-valve so you are probably on good footing to change it out on principal, and maybe that'll do the trick with your peculiar heat problem.

FastLane has them. Couple of days max to receive.

Nic
'85 300CD @ 143k miles