My 1982 300SD was running hot, so last week we repaced the radiator. Everything was perfect for about four days, it ran at a consistant 90c. Then started running hot again, over 100c always ( idle,slow,highway).
We replaced water pump, fan clutch, thermostat.......STILL runs HOT.
Pressure from overflow is excessive, but coolant, hoses, engine etc. are not excessively hot to touch, but the temp indicator is reading 100c plus. Also not getting heat inside car during test (just cool air)

 

When the engine is cool, you might try removing the upper radiator hose from the radiator, and then filling into the upper end of the hose until the upper part of the engine is completely filled with coolant. These engines had a tendency to trap an air bubble if you try to fill only through the overflow container.