1989 260E.
I had a difficult time getting through smog by playing with the mixture with a long 3mm allen.

I replaced the O2 sensor and back probbed it. At idle the O2 sensor stays low or high depending on the mixture adjustment. For example a small turn CW and the O2 sensor stays around 0.7v a 1/8 turn CCW the sensor stays at around 0.1V. In either case with the car under load or just keeping steady on the road the O2 reading always hovers between .5-.9V. Why doesn't the O2 sensor fluctuate more? If I create an air leak the O2 sensor drops as expected and if I starve it of air the O2 sensor goes to the high side.

There is an 8 pin connector in front of the battery (no LED in it) but the #3 (signal) does not have a connector in there! therefore can't bother with measuring duty cycle.

The car is a terrible challenge to get started if i set the mixture too lean. If set richer it starts but does take a few tries and needs a few minutes to run properly without stalling.

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1972 450SL
1982 300D Turbo