regarding the trap oxidizer..not on all cars...only put on the California versions...stay with the Federal version...they only have an EGR valve.

I will have to say that I am not very familiar with all the changes, but my understanding regarding the 84-85s is that they have an upgraded vacuum pump..piston type. In the older diaphram type, the diaphram could break allowing oil to travel via a vacuum line into the fuel injection pump...dirty oil in there would wreak havoc..if buying an older 123 you must check this.

I also understand that the 84-85's had improvements made in the tranny.