smog question-failing nox

[Big Red]

I'm trying to sell 95' stock cherokee and am having a problem passing on the 15 mph nitrous oxide #'s. I failed and then bought some "guaranteed to pass" stuff that I put in my gas and then drove 300 miles round trip to Redding only to fail the test again. The only thing that changed is my 25 mph nitrous oxide # went from 500 down to 150, but this always passed. It's my 15 mph nitrous reading that I keep failing on. I need to have under 517 parts/million. I had 514 when I bought the jeep this june and just barely passed, when I failed it was 546 and when I added the "guaranteed to pass" crap to my gas and ran the 300 miles trip it is now 614. The smog guys said this is the temp of my pistons and I probably have some carbon buildup. I have a guy lined up ready to buy the jeep from me and I just need to pass smog. I was also told that if the co# goes up the no# will come down. This is what happened to my 25 mph no# that went from 500 to 160 but the co# went up from .01 to .65 but I still passed barely because the limit is .80 What do you guys suggest? Also what is a good smog shop in the NE Sac area (Citrus Heights) I'm looking right now at Smog Diagnotics, Earth Station, and Goodyear Gemini.

 

[Phil]

Put a couple twenties under the hood for the smog check guy? :dunno:

Yes, this post is worthless.

 

[Bryan C.]

NOx failures are difficult to diagnose and repair properly. Carbon buildup on the pistons and in the combustion chamber is fairly common and easily repaired. Go to your local GM dealer and purchase some of their Top Engine Cleaner and follow the directions on the can. This would be where I would start.

There are other causes of NOx emissions, an engine that runs too hot or on the verge of overheating, if the engine is running too lean. Has the head ever been resurfaced? Make sure all the sensors are operating normally, especially the O2. I recently had a 94 ZJ 4.0l fail for NOx at work. What I found was a partially restricted exhaust. This caused the MAP sensor to give a false load reading to the engine controller. This made the timing advance excessively under load and caused the increase in NOx. The repair was to have the cat and muffler replaced. Once the sensor readings were in line, and the new cat helping to clean up any leftover emissions, it passed easily.

I wouldn't suggest just installing a new cat, although it has been done many times to pass a smog test. Having to install a cat every 2 years to pass smog can get expensive.

Start at the basics. Once those are all checked out, you may still need to have it diagnosed by a professional.

Bryan

 

[Big Red]

Thanks Brian. The smog diagnotic shop said the cat & O2 sensor is bad. I'm having a new cat put in and smogging it.

 

[Bryan C.]

Glad to hear you are making headway on the repair. What about tires? Are you running oversize tires and stock gears? I thought about this and wondered what you are running. It may make some difference.

Good luck
Bryan

 

[Big Red]

The jeep was bone stock except for the herculiner I put in it. I passed smog after having a new cat & O2 sensor installed. Thanks for all the info. The jeep is sold.