My '89 300E was only blowing heat when I would be at higher RPM's. It would blow cool air at idle or lower RPM's.

I replaced the circulating pump and it worked better, but no cigar. My indy unhooked the mono valve plug (don't know the name of it) and it defaulted to hot air--full blast!

He then unscrewed the mono valve and took out the metal part that looked like a piston and cleaned it and the heater has been working great at idle and all speeds.

There must have been some tiny particle inside the mono valve that was hanging up because it wasn't the diaphram.

BTW, I now have 204,000 on my car. Any idea how long these things run? I've read they will do over 300K if properly serviced. I bought it with 193K over a year ago and I just love this car!