Hello

I have been reading everything I can find on heating and it all seems to be reversed of my situation so I have to ask.

I have a 81 300SD it will burn you out at road speeds with no problem but when you slow down like in town to say 30mph or lower or stop the heater will cool down to freezing you out.

I have just replaced the aux water pump.
I have reworked the control unit and have even replaced with a known good one.
I also just flushed and filled the antifreeze while I did the aux pump.( And I used the Zerex antifreeze).
The engine temp stays at normal. ( And always has ever since I have owned this car)

The blower motor runs like normal not dragging or anything.

Also if you let the car idle with the blower on low you can get very little heat out of it. But not enough to stay warm.

I was thinking of the mono valve but I am not sure as all the articles describe just the opposite problem for that.

Any help will be greatly appreciated.

Thx
Ken