I have an '87 300SDL 308,000 miles. I had a gradual rise in operating temperatures until it operated at 100 degrees C after a mile, down the road. Was not the thermostat, waterpump, leak, lack of coolant, clogged catalytic converter. It was simply a clogged radiator. Even if the coolant is clean and clear it doesn't mean that your radiator is not clogged Try the citric acid flush of your radiator. You should find that the temperature going up a steep hill, 90 degree outside temperature and the a/c on shouldn't bring you anywhere the 100 degree mark after the flush. I've had two other 300SDL's and two 350SD's all have suffered from the same overheating malady and all needed the same thing, the citric acid flush of their radiators.

'87 300SDL 308,000
'89 300TE 176,000
'94 SL500 55,000

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jcciem
'94 SL5000 60,000
'91 350SDL 545,000
'91 350SD 197,000
'00 E55 AMG 77,000