Failed Emissions

1993 Grand Cherokee 4.0L with 234K miles. My Jeep failed the Nox portion of the emission test. Does anyone possibly have a solution? My Jeep is running better now than it has for the last year. I have replaced virtually every sensor on the engine, plugs, wires etc and even installed a new cat converter afew months ago. The O2 sensor was also replace. Any suggestions????? Thanks, Mark

high nox can be caused by a couple of things,but usually it relates to high combustion temperatures.The easiest way to reduce nox is with an egr valve,but I believe 1990 was the last year the 4.0 had one of those.ANother thing that could cause it is over advanced ignition timing.That should not be adjustable on your jeep,but I have seen people do it.The only other thing I could think of is if the jeep is running too hot,that can have an affect.

Where is your temp gauge needle at when it comes up to temp?

I will have to double check that but I believe it is at 180-200.

Seems right. I stay steady one line under the 210.

What plugs are in there?

My records on that are at home, I will have to check and get back. However, I did pull all six plugs two weeks ago just to spot check and they were all good and burning clean. It seems they are a AC but I can't remember the number. They are the same type I have always ran in the engine and I never had a problem passing emissions before. I am required, here in Wisconsin, to pass emissions every two years. Thanks, Mark

I have Autolite 985 in mine. I just passed an emissions in Mass.

The interchange charts are saying lower numbers mean cooler for Autolite (that is not always the case mfr's are different) so I'm wondering if you take what xj31 says and find the next step lower. Pop them in and go back for a retry.

Take them out after if you want and save them for inspections. :iceslolan

Just make sure you have adequate fuel pressure and no lean conditons since everything else looks ok. I would suggest try another cat if everything else looks good.

SOLVED........it was the catalytic converter. I had bought a CATCO catalytic on line a year ago and installed it on my Jeep but that catalytic would not pass emissions due to the fact it did not have a Nox bed in it to reduce the Nox emissions. No wonder it was only $50. Wisconsin starting checking for Nox only two years ago so most aftermarket catalytics will not pass in this state. Thanks guys for all the help. Mark