Originally Posted by Duke2.6

Turn the fan by hand when the engine is cold. Then, again when the temp is over 100C. It should be quite stiffer. It sounds like your fan clutch is not functioning properly. Some have added silicone fluid that you can buy from hobby shops, it think, but if the seal is bad it will likely leak out very quickly.

Also, if the fan clutch is good you should definitely hear "fan roar" if you rev then engine to 2000 or more. If you don't hear fan roar at over 100C the clutch is bad.

When cold the fan should keep spinning a bit after engine shutdown. At over 100C when the clutch is fully engaged it should stop quickly.

While idling the mechanical fan doesn't flow much air, even if the clutch is fully tightened, which is why these cars have auxiliary electric fan(s). (My 190E 2.6 has two that are smaller diameter than the single electric fan on your 300E.)

Running the A/C in low speed stop and go traffic actually aids cooling because the electric fan(s) engage at low speed from a A/C system pressure switch, and temp may not reach 105C when they engage at high speed.

In hot weather stop and go traffic once the temp get to 105C and the electrics engage I can actually see the temp gage needle moving down to when they shut off at 100C. Then the temp creeps up again fairly quickly and they engage again, so the "duty cycle is probably close to 50 percent.

That's typically how modern engine cooling systems work, however, especially on FWD vehicles with "sidewinder" drive trains, there is no mechanical fan... just one or two electrics. The other issue is that most modern cars don't have temp gages, only an overheat light, so most owners have no idea what coolant temp actually is and may even be oblivious to failed electric fans until the "engine temp" light illuminates or coolant is expelled and steam rises from the engine compartment.

Duke