Every time I see people say "Oh yea, high NOx is a cat" I cringe.
NOx is created by HEAT when the cylinder fires.
NOx is reduced by the front half of the cat. If the front half of the cat is being fed too much O2 (flooded), it will not work.
ANY exhaust leaks before the cat will cause this (Main problem with our Jeeps).
A Biased rich O2 will fool the computer into running leaner causing too much NOx (#2 issue with ourr Jeeps).
A LEAN mixture will create extra heat and NOx in the process.
Spark plugs with the wrong heat range will cause high NOx.
Ever replaced a freeze plug, couldnt get the old one out so pushed it into the block (or thet lazy mechanic at your local speedy tune)? You just created a hot spot in the cooling system, and up goes your NOx...
Gear ratio change? (I went from 3:07 to 4:10 and caused a BIG NOx issue with my '94 4.0L AX15) you could easily put your jeeps load vs speed into a NOx heavy powerband on the smog dyno...
Excessive carbon on the pistons and combustion chambers will raise compression, the carbon will hold heat and cause pre-ignition and the result is... High Nox.
And LAST, a damage cat will also cause high NOx
HC is un burnt fuel, raw gasoline, a misfire to be blunt.
HC is a direct result of fuel not burning, and a tune up is normally in order.
If you do a tune up, PLEASE use stock Champion Copper spark plugs, too many people get "upsold" into double platinum by the parts "professional", just dont fall for it, use the Champion cheap plugs.... Trust me...
I live and work in Cali, we measure our tailpipe gasses by PPM instead of % (except for CO, and CO2), and the translations can be odd, but the gasses are all the same, and gasses that are too high have the same repairs no matter how you measure them.
Originally Posted by wcjeep
It's probably the cat.
However, your Jeep uses a forced orifice instead of an EGR Valve. There is usually a plate below the orifice. The plate would be inside the valve cover. Cheap oil and/or poor maintenance can lead to lots of crud developing on the plate. The crud can block the orifice. This is usually an extreme problem.
This is wrong on 2 levels,
No, the injected 4.0 has no EGR.
An EGR will directly reduce NOx (it is its primary function).
Any EGR system will NEVER go to the valve cover... One end goes into the intake manifold, to full vacuum, the other goes to the exhaust, with a ported vacuum line on top to open it up under load
This orifice you speak of is the PCV, a totally different emissions system...
The PCV system purges gasses that escape through the piston rings during compression, and sucks them out to be re-introduced into the air/fuel mixture...
Originally Posted by DREDnot
....snipped out a part to maintain my sanity... The 94 uses a THREE WAY cat. (HC/CO/NOx)
....snipped out another part to maintain even more of my sanity... They are more expensive, but the cheaper two way cat wont fix your NOx.
You would be wise to change out your O2 sensor, and tune up parts(plugs, cap, rotor, wires, filters) before trying again.