@jastutte The fan clutch is only controlled by temp - so if it gets hot under your hood it engages even at highway speeds. Most start to lockup at around 170F (180 to 190) engine temp, the clutch picks up waste heat from the rad for the most part to activate, add A/C and a hot humid day and higher RPMs under load it will engage

 

Confirmed, the nail in the coffin is the Temperature Sensor giving incorrect readings. I found the original MEK100160 sensor, cleaned it up, tested it with boiled water, reinstalled it, topped off coolant and went for a ride. Ambient temps were 75-80F and raining, I will make another run in the ridiculous heat tomorrow..

Note I lowered the minimum temp of the graph to 170F


Factory MEK100160 sensor



Delphi sensor

 

That's gotta feel good to have figured out! Nicely done!

 

It does but it's ridiculous that 2 other sensors are completely wrong.. Blatant Chinese knock-offs of things work better than these "aftermarket/OE quality" parts. A Temp sender isn't a crazy new device, it shouldn't be that hard for these companies to get right.

 

I just started digging into BMW 4Pin ECT Sensors to compare, trying to matchup which BMW used the Motronic 5.2.1 ECM.
WALKER PRODUCTS 2111053
NTK EF0082
CTS-066
BMW > 1997 > Z3 > 2.8L L6 > Cooling System > Temperature Sender / Sensor