I had the exact same problem with my 87 300E and I replaced the thermostat and nothing happened. The way to tell if your fan clutch is bad is to rev the engine and have someone watch the main fan. It should speed up in relation to the engine speed. If the clutch is bad, it will just barely turn while the engine is doing high RPMs. After changing my thermostat, my tech wanted to see if the aux fan would cut on, so we parked it in the garage with the AC on and let it idle, we even put a sheet over the front of the radiator so as to block off air flow, and we could not get get the temp above 110. the aux fan will not cut on until the temp gauge hits the white line directly below the red line, my aux fan still worked, he connected the wires to make sure it ran. In conclusion, we found that we could not make my car overheat, even though it runs at temps of 95 to 105 regularly, so we decided i just have a 300E that runs warm. It is not harming anything, so oh well. Hope this helps.