i have a 1990 cheyenne stepside that came stock with a 4.3L
i put a 95 5.7L in it and went to change out computers and it would not fit
is there anything i can do without having to replace the wiring harness that would fix my problem of it running open loop and make it go closed with the existing wiring

 

not likely. mainly cuz you went from six to eight cylinders, jumped up five years newer. you couldget an entirley new fuel injection system from the aftermarket area. something like edelbrock or holly do work well. problem is that it is expensive. if you coulod find the same ecm and wiring harnessfor the new engine you would be better off. just a little simpler less stress. sorry i cant be of more help