I have not been posted in a long time because my 1984 300SD has been running so well and any repairs I did were easy. I must say that this heater problem has really confused me.

The engine warms up normally. When the car is idling or moving below 30 mph, the heat is very good. The temperature of the heat at these speeds is about 120 degrees. When the car goes faster than 30 mph, the heat temperature rapidly falls.

The same problem also happens when standing still. When the engine speeds up, the heated air temperature drops. I must mention that the engine temperature remains constant at all conditions.

My first attempt in trying to solve the problem was to find out what happens when the monovalve is disconnected. I removed the connector from the monovalve. The blower blows much warmer air, however, it blows hot air when the climate control is in the cooling mode. This is expected since the monovalve is wide open when the wire is disconnected. I also inspected the monovalve diaphragms, and they are in good condition.

I also disconnected the auxiliary recirculating pump. As I understand, this pump increases water flow at low speeds. The result of disconnecting the recirculating pump was that the air got hotter. This was unexpected. I was expecting that a non-functioning recirculating pump would reduce the heat. Some members said that the recirculating pump was not necessary since it was needed for more frigid weather.

I also flushed the system. I used Prestone Super flush. I drove the car for 2 weeks with the flush in the radiator and then flushed the system with lots of water. There was a little oil in the system, but otherwise, the system was clean. The flushing did not help the problem.

I also checked to see if any heater hoses where collapsing at high rpms. All the heater hoses held their shape at high engine speed.

Radiator cap is holding pressure. Water level is at upper limit in the expansion tank. Antifreeze is 60/40 Peak Global, the yellow stuff.

I noticed that other members have had same type of problem, but I did see any posts saying that they found the solution.

What advice do you have?

Thanks.