I just bought a 91 Chevrolet, 5 speed, 4 x 4. The original computer was not with it. The previous owner had put a computer in it from a van. It kept gas fouling the plugs. After changing all the sensors and distributor, it was still gas fouling the plugs. I put a hyper tech performance chip in and it seems to be running better, not fouling the plugs, shift light stays on and cuts out when shifted into overdrive. Gears 1 thru 4 run fine. Any suggestions about the overdrive?