Erubin, I am curious. Since your duty cycle is at 22% O2 sensor disconnected, why don't you just adjust the mixture to 22% with your original meter in close loop? Older MB apparently is different in terms of fuel mixture design, using a "calibrated" equipment certainly entailed guessing, I think.

I don't know how to disconnect my 85 500sel's O2 sensor, I couldn't find the connector under the passenger side carpet. Can you tell me how?

Same as your finding, I couldn't lean the mixture above 46% or it stalls after that. I couldn't read duty cycle KOEO, either. The voltage is 0.05v by measurement at pin 3. I am very intrigued after reading various posts including yours. After some experiments, it seems for our car, the duty cycle somewhere around 22-25% is the sweet spot.

My car runs great. I don't think there is anything wrong with it. What do you think?