I have a '95 3.8 Mustang. It runs really rough after about 2-3 minutes of starting it until about 15 minutes of driving. It is only rough below 2500 rpm. I hooked up a scaner to it and noticed that O2 Bank 1 Sensor 1 was giving wildly varying voltages from .05 to .78. I assume this is a bad O2 sensor and that it would cause the described problem. Also which sensor would that be? I think
Bank 1 are the ones on the manifold, but which side is Sensor 1?

 

hi, bank 1 should be passenger side, and sensor 1. it the upstream sensor on that side.

 

O2 sensors are supposed to jump between 0.2 & 0.8 volts in a sin wave (yeah, math). So your O2 sensor is fine.

With your scanner, check if your problem occurs when it is in closed or open loop mode. Probably open loop, in which case the computer isn’t even listening to the O2 sensors and is getting its fuel trim numbers from the Mass Air Flow sensor, engine temp sensor and a couple others. good luck...