Excessive NOx 93 4.0 h.o. CA XJ

So my gf's 93 HO 4.0 XJ failed the emissions test here in Oregon due to excessive NOx. It is originally a CA Jeep, and the previous owner put on an aftermarket air intake, but the motor is otherwise stock, 210k miles. It runs strong and doesn't have any other problems. I know that excessive NOx is due to excessive combustion temperatures and all of that. I was expecting to find an EGR valve, but I guess the 91+ 4.0's don't have one. Did the key on/off etc and no trouble codes came up. I'm sorry but I don't have the numbers from the emissions test showing how much it failed by. Also, I couldn't find any vacuum leaks.

I've read a few posts around the net about people having a hard time eventually finding the cause, even after replacing some parts such as the cat. Anyway, just wondering if any of you had any similar experience or solutions, and if any of those "guaranteed to pass" fuel additives actually work or if they would pertain to NOx problems. Would the aftermarket intake be causing a lean condition? Any help would be appreciated.

Excessive NOx is usually caused by a lean burn condition. I would certainly suspect your aftermarket intake, although I don't understand why the computer isn't able to compensate for it. How old is the O2 sensor?

We have had the rig for nearly two years, so I wouldn't know. The more I searched around on the site, the more I found that on non-egr'd 4.0's, the most common problems are a vacuum leak, too hot of plugs, or a bad cat. I have read that a bad O2 will most likely cause a rich condition, but it is still something I should look into tonight. It needs new plugs anyway, so I'll try that first. Obviously the cat will be the last result, for money reasons. Still curious if any of that snake oil emissions passer stuff would be worth the $12?

Not to disagree with Eagle since he really knows his stuff, but I don't understand how a properly installed aftermarket intake could effect your NOx levels or make it run lean.

So I pulled the plugs tonight, and sure enough the previous owner was running Bosch Platinum plugs, which were as white as could be with tons of corrosion/build up. I replaced the plugs with some stockers. The DEQ test is free if you fail, so I'll go run it tomorrow and see if it was just the plugs.

i live in CA and had my 94 xj last year fail smog last yr due to very high nox values. I had someone look at it and they said it was due to a faulty o2 sensor . THey replaced it and passed it. Not sure tho if it passed due to the o2 sensor or rather them taking the money for the 02 sensor plus labor...lol..what labor i know...but it passed.

I would try the 02 sensor.

pete

I had the same high NOx problem a few months ago on my new stroker engine. I put in one step colder than stock plugs and it passed.

Changed the plugs only, and it failed again due to Nox. I don't have the previous numbers, but this reading was 4.59, the max allowed is 3.75.

does it have the manifold leak that most do? A leak upstream form the cat will also cause High NO2

Damon Dimick
please post what happens.... my "90 is failing on nox and i replaced the cat. but it still failed. im running bosh plugs. people have told me that they run too hot. i might replace them with some chanpions.

A lean condition could theoretically be caused by an intake modification. Letting more air in could lean the mix, considering it's a speed density system that doesn't use a mass air meter to actually measure air flow, BUT the computer should be able to richen up the mixture from the lean signal coming from the O2 sensor.

Something else to consider is your fuel injectors. With that many miles it's possible they're producing an eratic spray pattern giving you a lean burn. I also agree that a bad O2 sensor usually results in an overly rich condition.

Eagle said:

Excessive NOx is usually caused by a lean burn condition. I would certainly suspect your aftermarket intake, although I don't understand why the computer isn't able to compensate for it. How old is the O2 sensor?

Click to expand...

High NOx=Lean condition
The computer should be able to increase the injector duty cycle to compensate for the increased airflow from the aftermarket intake. The problem is that if the injectors are old, you may not get enough juice out of them. An injector cleaner might help, otherwise you're looking at a set of new replacements.
A bad O2 sensor usually causes a rich condition but if there's a short in the wiring to the sensor (can happen if it's been burnt by the manifold), you could get a lean condition instead.