I been doing some work trying to get my 240 running right
along the lines I changed the single wire O2 sensor in the engine bay, but I saw this :


What is this connector for? I tested it and it's getting 12v
It's coming out of the fire wall along with the 02 sensor wire what could it be?

 

That is the power connector for the heater for a 3 wire o2 sensor. Those first appeared in late 1984 from what I remember. The heater warms up the sensor quicker because O2 sensors do not produce the correct output voltage until they are XXX degrees. So your ECM is programmed to not accept the signal from the O2 sensor for about the first 90 seconds after starting.

If you are "trying to get my 240 running right" - in that year range - simply leave your o2 sensor disconnected. The ecu will use it's stored "map" and the car should run just fine. (unless there is another problem!) Early/mid 80's o2 sensors did very little. The o2 sensor is there to make the cat converter be most efficient. At this point is there still catalyst material in that can?