Just guessing here, but the aux. fans should be on at low speed nearly all the time, even at idle, and is what normally keeps your car in normal operating range.

If they are not on, then your car quickly goes up to higher temps, and then the high speed engages, which saves your engine from overheating.

I'm not saying your viscous fan is bad. Could be.

When I owned my 1993 W124 300E (3.2) "E320", I had the same overheating problem, took it to Enrique, and he showed me that the fans weren't engaging at low speed.

I thought it was my viscous fan, but he showed me that it was working, and explained that they rarely go bad.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".