I don't know how you get the idea that the ECU doesn't use any input from any of the other sensors. IOW, the ECU constantly needs input from the other sensors, ie, coolant temp sensor, TPS, AFM to include air intake temp sensor, etc. But, as far as being in open loop because the O2 sensor is unplugged, your right. The ECU won't go into close loop under several conditions, one of them being if the circuit connection to the O2 is lost. But, the theory is if the O2 sensor input is faulty, then the vehicle will run better if it's no longer affecting how the ECU is controlling EFI operations. IOW, from what I've learned, if there's only a problem in closed loop mode.....anything disrupting air/fuel balance......then, the O2 sensor is probably the fault. (This also being from what sb5walker has said.)

So far, by unplugging the Bosch sensor in my wife's '92, the mileage has gone from average of 14mpg to 16-17 mpg. Plus, the vehicle generally seems to be running smoother under would be closed loop situations. Now, I'm no expert, but being in open loop and running a richer would theoretically give poorer mileage than it would with a properly running O2 sensor trimming fuel delivery. However, I'm getting better mileage. Not sure how to answer why just yet and I still have to get my injectors cleaned, but I'll come to some conclusions as some point soon, hopefully.

 

Did you ever fix this problem? I am going through the same issue.