I initially thought this was a fan clutch problem as well. I did the "newspaper" test--couldn't stop it when coolant temp in the 100 degree range. Could stop it when engine first started cold.

I have a couple old fan clutches laying around in my shop. Maybe I'll rig one permanently engaged, install and see what happens......

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1994 E320 Wagon, 230,000 miles
1995 E320 Sedan, 106,000 miles
1994 E500 Sedan, 79,000 miles