Re: oxygen sensor stepper motor
BigJ -- here's how I see it. The stepper and MCU are functioning , as os the o2 sensor. The O2 sensor will generate higher voltage up to 0.9 volts when the exhaust is rich. This voltage will inform the MCU of the rich exhaust status. In response to the voltage signal sent by the o2 sensor, the MCU will tell stepper motor "please lean the mix by moving pins to the extreme lean position , that is bury them to the firewall side ".
As you unplug the O2 sensor , you kill any voltage signal .
The MCU will interpret the lack of voltage as " exhaust is too lean " . In response to this lean interpretation , the MCU will tell the metering pins : please move to the full rich position , that is to bury them toward Rich radiator position side .
I tend to believe that O2 sensor is working, MCU is working, stepper is working.
I would try choking off the air way with your hands and see if the pins move toward the firewall. That would indicate that exhaust was perceived to be too lean prior to the choking off action of the airway . I would suspect a vacuum leak in the numerous endpoint switches /sensors that receive manifold vacuum .
Next, I would cause a massive vacuum leak by pulling a manifold vacuum hose . In response to this leak, the pins should bury themselves toward radiator. If they did not move , that would point toward a pre-existing vacuum leak.
If the pins are sitting buried to the firewall ( trying to lean out position ) , I would say that the O2 sensor is already sending high voltage due to a perceived rich exhaust and that the idle mix screws need to be turned counterclockwise inward to lean the idle mix.
Hope this helps.