damm RTH...you mind as well work for MBZ for the diesle line up..since you know so much already. I feel your pain, i have the same problem with my 300D 78. Well i manage to swap up some info from my mechanics office diesle training service manuals. I'm guessing that you cant buy these manuals anymore because they were only used for students attending the diesle college for MBZ. I'm not sure if this stuff will work but here it is:

Engine is Overheating:
1. Insuffeicient coolant in radiator.
2. Coolant circulation is obstructed.
3. Loose or broken water pump belt.
4. Thermostat defective.
5. Air bubbles in cooling agent.
6. Exhaust system restricted.
7. Injection timing incorrect.
8. Inferior fuel quality.
9. Cylinder head gasket leaking.

This stuff is taken directly out of the office mercedes benz service training books...and thats all it says about engine overheating, doesnt even say anything about normal ambient temperatures for the 300D/240D engines.

I've just got done flushing my radiator tonight and I used the BG products to flush out the stuff in the radiator. I took the 300D for a spin tonight (about 65F outside weather) and when i'm cruising around 45-50 mph the temp would stay about 200F. When i stop at a light, and wait for 3-4 minutes, the temp would past 212F line, just a little more than half way from 250F red line. When i get going...the temp drops back down to 200F, no less than that....damm there is something wrong here. do i have the incorrect coolant mixtures when the temp fluctuates like that? I didnt even have the A/C on the whole time.

Also after i flushed the cooling system out, I can hear these girgling sound from the top radiator hose, coming from the thermostat housing to the radiator....thats when the engine is hot...is this normal?

thanks for replies and i hope that some of the info helps out
Allan