more emissions

Took the 88 4.0 xj to get inspection the other day. last years results were great! Did An exhaustive search.
2003
Limits------------ Test

HC-----2.00---------------0.41
CO----40.00---------------2.24
NOx----4.00---------------0.92

This year 1st Test (limits have changed)
Changed mufler and cat high flow
3 weeks before test
Limits-------------- Test

HC-----1.60---------------0.15
CO----40.00---------------4.33
NOx----2.50---------------2.82

Changed air filter, cap & rotor(all fouled)
2nd test
Limits-------------- Test

HC-----1.60----------------0.29
CO----40.00----------------6.02
NOx----2.50----------------2.58

Cleaned throtle body, Jeep top end cleaner, New plugs,techtron injector cleaner( was in tank during test), oil & filter, and 1/2 the vacume set(front of valve cover, small 1 from rear was replaced 4 months ago) air box side( brakes are much better, and idles great! like new).
3rd test
Limits-------------- Test

HC------1.60----------------0.43
Co-----40.00----------------5.80
NOx-----2.50----------------2.48

It just passed! What do I need to do to make it closer to last year so I don't go through this again? I bought a new egr valve(not looking foward to installing), and O2 sensor have not installed yet. Should I buy the block temp sensor or anything else? Why is my NOx so high?
Thanks for any help.
Webster*

High Nox readings are the result of high combustion temperatures and/or high combustion pressures.
Possible causes are:

* Malfunctioning EGR valve
* Lean fuel mixture
* Improper spark advance
* Thermostatic air heater stuck in the heated air position
* Missing or damaged cold air duct
* Combustion chamber deposits
* Malfunctioning catalytic converter

My bet would be for the EGR valve. You can test it with a hand operated vacuum pump and determine if it's working or not. Sometimes they stick open or the vacuum diaphram become perforated and it won't open. It's not difficult to replace if you need to.

More than likely the EGR valve. Also if its been a while you might want to check out how your O2 sensor is working too. Poor grade fuel can also have an effect on your Nox, Higher grade and higher octane fuel usually will slightly lower you NOx

Thanks for the advice. The egr is working but I bougth a new one. Also got a new o2 sensor guess I'll be replacing both. The thermostatic valve is opening and closing(guess it could be blocked so I'll fish it out) and I usually use at least Hess gas or better. Any other Ideas are welcome.

Thanks,
Webster