1993 Ford Mustang Failed Smog Even After Repairs

Based on the VIR numbers you indicated we recommend you inspect your Ford Mustang's fuel control system. Your Ford Mustang appears to be producing high CO (carbon monoxide) emissions. High CO is due to rich fuel mixtures. We recommend inspecting the MAF (mass air flow) sensor. The MAF sensor measures the amount of air entering the intake manifold. This sensor must send accurate information to the ECM as well, in order for the ECM to calculate exactly how much fuel to add to the air entering the combustion chambers. Once air/fuel ratio is corrected HC will reduce as well.

As far as high NOx we recommend verifying your Ford Mustang's EGR (exhaust gas recirculation) system is working properly. The EGR system recirculating burned up exhaust gases back into the combustion chambers. This keeps the chamber temperatures down and thus reduces NOx emissions.

Inspect your Ford Mustang's EGR valve to ensure its proper operation. A working EGR valve should be able to open its passage using manifold vacuum. It may be possible that the EGR valve is plugged with carbon deposits and is not opening properly causing a exhaust flow restriction, or there may be a problem with the vacuum system which is used to open and close the valve.

To test the operation of the EGR valve attach a vacuum pump to the EGR valve while the engine is idling and attempt to open the valve manually using vacuum. Under normal conditions this procedure should cause your Ford's engine RPM to drop at least 500 rpm's (while the valve is open). The you see no RPM drop more than likely the EGR valve is plugged up. You should remove the valve and inspect it for carbon build-up. Be cautious while performing this test as the engine will be hot.