I failed smog on the NOx gas, but only under load. I drive a 1991. I am in auto shop and have done a compression test on 4,5,6 cylinders. My teacher is totally stumped!!! I got a top end combustion chamber cleaning and fuel injector cleaning (which my teacher thought would help), but that did not help. I have one year old cats and oxygen sensors, so that should not be the problem. I have the Kooks headers/ high flow cats and downtubes and the SCP cat back system. I also have a 10% OD pulley installed on the SC. The lattest suggestion made by my teacher is to put the stock pulley back on because the compression may have been changed. I don't have any other major mods. Im in california, and everything else passed except the NOx gas. Any other ideas or suggestions on where else to look??? PLEASE HELP ME, I need to get this fixed ASAP!!!