I had my 91 2.1 Si rebuilt recently. The block received a .020 bore and new pistons/rings installed. I immediately had error codes for the o2 sensor showing up. With a volt meter on the O2 sensor and a vacuum meter on the EGR. You can see the computer give up control after a few minutes driving. I believe that the displacement increased by about 2.5% and the compression ratio increase from 9.4 to 10.4 or about 12%. Could this have placed the motor beyond a computer set-point? Any ideas?