1991 420 sel heater problem?
 

Ihave recently purchased the vehicle and found the heat only working while car is sitting at idle,lots of heat! As soon as you drive down the road we get cool air. I changed the thermostat thinking it may help but no change. Is this something in the switching valve i've heard so much about? Any chance this could be part of a vacuum leak?

no heat when drining cause?
 

probably a vacuum leak in the actuator (vacuum motor) for the temp blend door

Check the Pods
 

I recommend you check the vacuum pods. There is vacuum distribution valve accessible through the front passenger foot well, it is up against the transmission hump. Get your mity vac out and see if all of the pods hold a vacuum. If one doesn't then you'll have found the leaking one.