I have a 1991 TA 5.0 TPI and had emission tested and failed just about everything but the gas cap test. I changed the o2 sensor and retested and failed. changed the egr valve nuber were better but still failing had the cat changed and passing everything but the nox and its not even close I check for vac leaks and found none I dont know what to look at next. nox numbers were 1700 allowable 796 and 2130 allowable is 879

 

NOx is greatly influenced by the EGR valve. The EGR valve is controlled by a solenoid that allows or blocks vacuum to the valve. You might want to check out the connections at the solenoid.

High combustion chamber temps cause high NOx readings. By recirculating inert exhaust gas (EGR) into the air/fuel mixture, the combustion chamber temps drop.

 

Thanks I didnt know about the solenoid. Expensive little buggers aint they