Good catch. I was thinking that even a bad monovalve allows heat when the car's not moving. I don't understand it but that's how it was on my 300SE. New cartridge fixed it.

tenmaz, remove the electrical connector from the monovalve and go for a drive. If that gives you heat at speed then then a monovalve cartridge won't help.

[edit] Oops, you already did that.

Sixto
93 300SD 3.0