weird heating issues.....
 

I have a problem with my heat in my W123 chassis 300D.


Here is whats is happening. Temperatue guage usually runs right at 100 degrees Celsius. Thats the normal operating temp for my car.... I have a rather new water pump(replaced less than 2 years ago), rather new thermostat, and rather fresh coolant...


When I drive the car around town, I get plenty of heat. But, when I hith the interstate, going up over approx. 50 MPH, the amount of heat coming from the heater core decreases.... Temp guage stays the same, but the heater just doesnt put out much heat at higher speeds......

There are no leaks in the system. I havent had to add coolant since I replaced the thermostat.....

Anybody have any ideas as to what the problem may be, or what I might look at first?

Heater
 

1984 300d Turbo 142,000
Remove Top Radiator Hose And Fill With Water Air In System

Try to observe if the air flow is remaining the same. If not, try bumping the fan up to full high while stopped or slow and then proceed to highway speed.

You may have a control flap that is blowing or pulling shut at the higher inflow air pressures at speed. It is possible that running at higher fan speed will keep the flap open. If that is the case, that will keep you warm until you feel like going in to deal with it inside the dash.

Ken300D

Your monovalve diaphragm is probably torn. Obtain and install the monovalve repair kit. The job takes about 10 minutes. The monovalve is located on the firewall in the engine compartment, just inboard of the battery. All you have to do is disconnect the electrical connector, remove the four screws on the top and pull the solenoid out. Then install the replacement part. Then you will be warm at any speed.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1YC1BU6YY1YQ0LU2T6&year=1983&make=MB&model=300-DT-001&category=R&part=Mono+Valve+Repair+Kit