Originally posted by psfred
The hotter the rad the greater the heat transfer. Letting the coolant get to 120C before turning the fan on saves fuel and allows greater heat transfer from the engine to the air. No risk to engine.
You go ahead and trust Stuttgart engineering that designed the engine to run at 75F summer temps. I would never continue running my engine if it ever hits that temperature. BTW, why do you think they designed the clutch to lock at so high a temperature when they program the aux fan to cut in at a lower temperature? That means the fan never locks up unless the aux fan has failed in which case you are in overheat country. If the aux fan hasn't failed and your engine is still hot then there is something seriously wrong with the engine. My point with this is that the VFC has a flawed design that even MB's engineers acknowledge.
As for saving gas, Engine management retards ignition timing when the engine gets that hot and sure you'll save gas, but your engine will be sluggish at best. My car is happiest/liveliest between 85C-90C.
But that's just my opinion...