I have been looking over similar posts and have not found a distinct answer to this problem. Just took my 91 in to smog in CA today and failed the 25 mph NOx portion of the test with just over 700 ppm. Went and had cat replaced with a CA 3 way cat and NOx increased to 1741 ppm at 25 mph. My other gas readings are HC of 1, CO of 0, and O2 of 0. The vehicle has no error light on, and is running without issue. Only 98k miles on her, have 33s with 4.88 gears. Any thoughts or experience with NOx doubling after a cat change? Anyone encounter these numbers and find a solution? Thanks.
My fail was a bad fuel pressure regulator on an 89 2.5 TBI. Was running really rich the old spring had broken and was allowing too much fuel to pass through. I replaced the parts it cleared right up. Passed next try.
High Nox is usually a lean burn meaning too little fuel while high hydrocarbons means too rich. The CEL does not tell you if you have an issue with a sensor on the Jeep. Do the key cycle in this post and see if you have any codes other then 12,33,55.
In general, NOx is created by high temperatures in the combustion chamber. Running lean, carbon build-up, running hot (cooling system problem), misfire, timing off - all of these could increase the NOx. Of course, you could also have got a defective cat. Checking for codes is a good place to start.
You are probably already aware that if the cat isn't hot enough (such as due to freeway driving for 30 minutes or so), it isn't going to work well.
You also might want to try a different smog test station. Some techs know what they are doing and do professional quality work, while others just do a sloppy job. A sloppy job can result in a fail. I've learned this first-hand.
Skip this if yours is carb'd, but if its fuel injected then I would replace the O2 sensor ($30-$40), and at least the temp sensor in the thermostat housing ($15). Mine failed smog, but a tune up, oil change, O2 sensor, temp sensor and new catalytic converter and mine passed with flying colors.
Current - 1993 Wrangler 2.5 - mostly stock
Previous - 1989 Comanche Pioneer 4x4 Long Bed/Metric Ton Pkg
4.0, AT, D44 w/ Trak-Lok RIP
MJDriver said...just do a replacement or chefk of just major exhaust components, air filter, change the oil, and just take a nice lovely drive on the highway for about 30-40mins to get the cat really warmed up. For me I just replaced my air filter, drove for a nice evening drive on the highway, went to a different inspection station (jiffy lube to be exact lol) and passed with flying colors. It just all really depends on your heep.
My 94 failed with too high Nitrogen... Still haven't figured that one out- everything else was within spec. I now can't renew my plates until I can get it to pass- thankfully my YJ is just a toy and not a DD so I have been just trailering to trails.
1994 Jeep YJ off road toy
1981 CJ7 SBC 350
2005 KJ Libertyt 4x4 CRD w/ 2.5" Frankenlift DD