So I've tried everything that I could think of, replaced several essential parts, and now I'm at a loss. I've failed my smog twice due to high NOx levels while under load on a Dyno Smog Inspection and now I am driving on expired tags. I'm hoping someone out there will have the solution to my Nightmare.

Info:
1994 Jeep Cherokee 4.0L
NOx levels are Super High (Meaning Combustion Temperatures are High)
All other emission levels are Great!!
My Jeep does not have an EGR (Not installed on 1994 XJ's)
Engine has only 15,000 miles on it and passed smog inspection 2 years ago when it had only 200 miles on it. It now has a brand new Catalytic converter, Crank Position Sensor, and Spark Plugs installed. The O2 sensor was replaced last year. Compression levels are 145-150psi in all 6 cylinders. (Stock compression specs are 120-150psi) There are no exhaust leaks and the Water Temperatures are Great. Spark Plug color is light on 4 cylinders and white on 2 cylinders suggesting a lean condition. (The injectors are original so it is possible that they may be partially plugged although the Jeep runs smooth)

What do you think is causing the High NOx levels? Any suggestions to pass smog?
Thanks for your help.

 

Move to Pa where we dont have smog testing... wish I had your answer man. I'm sure someone on here does though. Good luck!

 

try running some seafoam in your intake and gas tank. Will clean gunk out of your engine.
Change fuel filter?

 

Moved to OEM Tech:

 

Btw High NOX is caused bu high cylinder temperature. Is your jeep running hot at all?

 

It would be helpful if you posted your other numbers.

The 4.0 uses a speed/density setup for fuel control, and it’s easy to get the engine to unintentionally run lean; low manifold vacuum at the MAP sensor will cause the ECM to fuel and spark as if you were using a higher throttle angle. It can also cause the engine to run lean, which can cause localized heating in the combustion chamber.

There could also be an issue of carbon buildup in the combustion chamber, which can cause high NOx due to an increase in compression, and the heated carbon itself can increase NOx during the combustion process.

I guess it could be an issue – spark plug heat range.

So your troubleshooting steps are to check manifold vacuum (leaks cause erroneous MAP reading), check fuel pressure, run a bottle of Regane fuel system treatment and verify ignition timing.

I don’t know if there’s a way to get the trans to kickdown earlier… you’re in California right? If so, doing the smog test in a lower gear will lower combustion pressure and reduce NOx concentration… I have used this trick before on Hondas (by adjusting the kickdown cable).

One last thing… are you running a free-flow intake/exhaust? It can exacerbate the problem.

 

Good suggestions. 00Pajeepcherokee-Moving to PA would certainly make it easier for smog. Metmknice-I did try the seafoam trick already and it didn't help. Probably because my engine has such little time on it after my rebuild that there was very little deposits to remove. JeepingDan-My Jeep is running nice and cool even though the combustion temperatures are too high. Thanks for the ideas.

blasto9000-I will post the other numbers Wednesday when I get back home. Hopefully those will help give you a better indication of what is going on. Should I run a vacuum test at idle or at some other rpm range? There shouldn't be carbon buildup, but I guess anything is possible. The spark plugs are the factory plug and heat range. I will check the fuel pressure and ignition timing. The trans idea is something I had in the back of my mind. I should throw some factory size tires on it and give it a try. That will certainly decrease the load on the engine and reduce combustion temperatures. The intake is factory but I am using a K&N filter. The exhaust is free flowing but just as it was 2 years ago when I passed smog.

Thanks for the help and the ideas.