We understand your frustration. Our only recommendation is to revert to the stock exhaust components and retake the smog inspection. It is very possible that the performance exhaust modification you've performed, although California emissions legal, is causing exhaust back-pressure to decrease.
Without back-pressure at required specifications the mass air flow sensor (MAF) and manifold absolute pressure sensor (MAP) might be sending data to the engine control unit that is causing a leaner fuel mixture than required for proper combustion. It will be hard to determine this unless you have a performance modification shop conduct an accurate diagnosis via a dynamometer and 5 gas analyzer. This would be our best guess in this situation.
The difficulty in determining that in fact your Mustang is producing high HC due to lower back pressure is because you're probably seeing really good CO numbers; which we use to determine rich or lean fuel mixture. Your CO may be at 0.00%. You'd think this is great. However too low CO may be hurting by the way of high HC. 14.7:1 air/fuel ratio must be preciously met in order to burn fuel efficiently. If you have the means, reinstall the exhaust equipment and retake the smog check.
posted by SmogTips Support 06-15-2017 10:10 AM