I just ran across this thread while researching NOx issues after my 91 Explorer failed emissions today. I know this is an old thread, but I thought I'd post to it to see if anyone responds. If not, I'll start a new one in a few days.
My research indicates that I don't have an EGR valve, and I have visually confirmed that. This was a pretty quick failure. I bought the Explorer last August. It came from a county that doesn't require emissions, so I had to have an emissions test before I could register it I'm just outsife Houston, TX in Harris County). Everything was fine in August. I've dumped a lot of money into it already in the short time I've had it, and I'd really rather not replace the cat converter without knowing that is the issue. Here are the numbers from last year and today. The ones in parentheses are last year's numbers:
H.S. STANDARD ACTUAL L.S. STANDARD ACTUAL
HC 143 16 (37) 147 39 (86)
CO 1.60 0.22 (0.32) 1.24 0.24 (0.42)
CO2 14.7 (14.1) 14.8 (14.1)
O2 0.2 (0.1) 0.2 (0.1)
NOx 905 1235 (358) 990 2252 (445)
Dilution >6 14.9 (14.42) >6 15.0 (14.52)
It runs great. No smoke. Very smooth. It has the towing package, and runs ver cool - just above the bottom end of Normal. Most of the work I've done on it has been brakes, oil change, air filter, trans filter, and sound system. It's a basic, dailt transportation vehicle. I drive 35 miles each way to work in heavy traffic, and we travel a lot on weekends across the state.
Any thoughts? I'd be happy to provide any additional info.