Mercedes: Heater/climate control, radiator fluid, coolant recovery

Question
I'm having a problem keeping the heat on in my 85 300D.  When it idles, the heat works fine.  However after getting on the road, it cools off, as if I'm turning the temperature down.  Could this be a thrermostat problem??

Answer
Yes the thermostat could be hanging in the open position. The engine fan in not bringing as much air through the radiator at idle. When at road speed more airflow means cooler radiator fluid.

I would also check the cap pressure. To get the radiator to maintain a higher temerature, the cap allows the pressure to build to 18lbs on some caps. You would need to ask the auto parts store what cap is required. Make sure the seat in the radiator is also clean so the cap will fit properly. The overflow tube must be in good shape to the coolant recovery tank. this tank holds excess fluid and allows the radiator to burp out air pockets so you have only coolant circulating.
Finally, the coolant mix must be proper for the vehicle. Usually 50%. Again check your owners manual for the mix.

Depending on how long the fluid has been in the car it may be time to have the system boiled out. There are products out there to help with this process, but it requires that you completely drain the system before you place the cleaner in. Then you use a pressure attachment to flood the system with a water hose before replacing the fluid with new stuff.

Good Luck
Rob