O2 Sensor
 

Just replaced my o2 sensor in my 86' today with 66,000 miles
on the original. What a difference! The car use to rev up when cold and now it is smoooth! What exactly does the o2 sensor do that would make such a difference?

The O2 sensor signal is monitered by your ECU which adjusts the fuel mixture accordingly. A low band sensor reports a voltage of 0V to 1V for rich or lean. If the O2 sensor has failed it probably reports as 1 volts. (no resistence). So the ecu thinks the engine is lean and supplys more fuel. Someone correct me if I am wrong on this answer!