Heat at idle, not on load or at speed - w123
 

Never had this before. The weather just turned here so I have the heat on.

On my 1985 300d, at idle: more heat than I can use. Temp gauge shows normal.

When driving: faster I go, less heat I get (have to turn up temperature wheel to staywarm). But car runs a bit hot, around 100.

At highway speeds the heat wants to go away completely. Have to turn the wheel way up (and operating temp does drop a bit).

This is the opposite problem I had with all my SDs, in cold weather.....

Low coolant in the system is the first thing to check. Second thing is a torn Mono valve. Third thing would be an inoperative aux water pump.

That's the classic symptom of a torn monovalve diaphragm.