NoX not Good

[zhanks]

My 94 Cherokee SE I6 4.0 with 160K mi on it is failing the TX emisions test. All items are way below normal except Nox. Passing is 1000 and my Jeep is running 1400 or so. I replaced the rotor, Rotor cap, Wires, Plugs, Oil, Oil Filter, I have a K&N air filter I rechaged prior to testing. I also put in a bottle of RxP just for good messure.

It runs smooth as glass and has tons of power so I don't think the cat is to blame. All hoses are in good shape and connected. I have heard the EGR is to blame for this usually but my engine does not have one and according to the firewall never had one.

What am i missing?

[zhanks]

I have not changed the O2 or MAP sensors. These send fuel air mix information to the computer. Could this be the problem or would I see increased CO and HC if they were bad?

[fredjacksonsan]

Google is your friend:

NOX failures are usually EGR-related, since the EGR system is primarily responsible for reducing oxides of nitrogen. But NOX emissions can also be caused by a bad three-way converter or a computer control system that remains in open loop.

Found on: http://hostingprod.com/@aa1car.com/library/tr1196.htm

-edit-
I wouldn't rule out a bad O2 sensor as a contributor either. Thing about older vehicles is that all the little things add up, so you may have more than one issue causing the failure. How's your MPG? If lower than usual, the O2 sensor could be contributing, but that would make the CO, I think, too high.

I've heard of people putting some alcohol into their gas, which supposedly cleans up the emissions for awhile. It's worth a try, some 90% rubbing alcohol from the local drug store should work, and much cheaper than buying many parts.

[zhanks]

My Jeep does not have a EGR system. What is the three-way converter?

[fredjacksonsan]

Google it, there are many sites. Even though your cat has good flow-through, the chemicals that perform the pollution reduction could have been reduced, over 160K miles, to a level where they aren't as efficient.

[89ltd]

i checked my 93 manual and it shows an egr valve, under the throttle body bolted to the intake manifold. Maybe they stopped putting them on in 94.