As mentioned, 82* C is the optimum but it's fine as long as it doesn't go into the red, if it does turn the heater (NOT the defroster !) on on maximum and open the windows, you'll bake but this should get the engine temps down enough to drive it home or to the shop .

Not sure why the thermostats fail so often but they do ~ mine is only a couple years / 45,000 + miles old and it's cold in the car, I had to run it hard up the mountain last night to get it up to 75* C so I could feel some heat in the cabin .

Doing a citric acid flush is really important every so often, these cars are closing in on forty years old and the accumulation of solder bloom is a normal thing, rarely is it necessary to replace the radiator unless it leaks or the inlet neck is cracked or broke off .

Doing a citric acid flush properly takes time, a thing no shop in the world will do .

Citric acid van be bought from Mercedes if you're rich, OnLine cheaply or at your local baking supply store, mix one pound per gallon of distilled water and run it for a few days of daily driving, you'll be shocked at the crud that comes out when you drain it .

Remember to flush the acid mix out and run at least one fill of baking soda and distilled water to neutralize all the acid as it remains in the heater matrix and nether regions of the water jacket....

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better