Cal smog question-failing nox

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.

Everything I've read in this forum and others points to the cat. Do a search here and at jeepsunlimited.com... and report back!

Oh the joys of double posting. I just replied to your post in the sierra chapter forum. Try to post questions in only ONE forum so you can avoid getting duplicate replies.

http://www.naxja.org/forum/showthread.php?t=17221

Bryan

Excessive nitrogen oxide emissions are due to high combustion temperatures that the EGR valve attempts to lower by injecting exhaust gases into the intake stream. While I'm not intimately familiar with the '95 HO engine, I suggest you check that your EGR is functioning properly--that the EGR valve is opening and closing correctly. Carbon buildup can obstruct the EGR port and the EGR transducer tube (if yours is equipped with one). There's also an electronic solenoid in the EGR vacuum circuit that might be failing.

Erik
88 Comanche

Thanks. The cat is bad as well as the O2 sensor.

Erik -
Just an FYI, but 91-up 4.0 does not have the EGR valve, it was deleted with the changeover to ChryCo electronics. The problem of NOx was addressed with an altered cam grind. This applies to XJ and MJ.

Don't feel bad about not knowing - I've got three Renix XJ's and it took me a little while to get used to the idea as well...

Emission notes - the Catalytic Coverter failure is mostly likely indicated by an increase in HC and/or CO emissions. The O2 sensor failure could lead to an increase in NOx by "falsing" a signal to indicate a mixture richer than it actually is - a lean mix will cause increased combustion chamber temperatures (production mixes are slightly "rich" to get a benefit from additional atomised fuel being vapourised by combustion chamber temperatures, which "uses up" a small but significant amount of the heat initially available when the intake charge comes in. This in addition to conductive cooling to surrounding air and to the engine coolant.)

5-90

5-90 said:

Erik -
Just an FYI, but 91-up 4.0 does not have the EGR valve, it was deleted with the changeover to ChryCo electronics. The problem of NOx was addressed with an altered cam grind. This applies to XJ and MJ.

Don't feel bad about not knowing - I've got three Renix XJ's and it took me a little while to get used to the idea as well...

Emission notes - the Catalytic Coverter failure is mostly likely indicated by an increase in HC and/or CO emissions. The O2 sensor failure could lead to an increase in NOx by "falsing" a signal to indicate a mixture richer than it actually is - a lean mix will cause increased combustion chamber temperatures (production mixes are slightly "rich" to get a benefit from additional atomised fuel being vapourised by combustion chamber temperatures, which "uses up" a small but significant amount of the heat initially available when the intake charge comes in. This in addition to conductive cooling to surrounding air and to the engine coolant.)

5-90

Click to expand...

Thanks guys for all your info. I added a new cat to the Jeep (OBD1) which the smog diagnostic shop recogmended and the jeep still failed. The shop did tell me that the O2 sensor is bad. I was hoping that the new cat would help me pass, but I failed really bad this time (my 3rd time trying to smog in the week). I thought a new cat would help me pass, but not make things worse. I will replace the O2 sensor and hopefully then I can pass. I'm coming to realize that a nox problem is very hard to diagnose and fix. At least I needed an OBD1 rather than the OBD2 cat. My engine light hasn't been coming on, but it did stay on for awhile when I 1st bought the jeep in June and then it just barely passed. The cat only cost $100 installed though. Is an O2 sensor easy to change out? Does anyone know how much it costs for a new sensor? Thanks again for all your help.
Troy

The OBD-I O2 sensor should cost you around $60-75, and you will need a specialised socket to change it (it's a long socket with a side milled out to clear the electrical connector.) You will want to change it with the engine still warm, as that makes it easier to pull the sensor. Expect it to take about a half-hour to change the sensor.

Changing the cat will have a negligible affect - if any - on NOx emissions. Changing the O2 Sensor will have a greater affect, since it generates a signal that is important to fuel/air metering... I'd also check the MAP sensor and IAT sensor, since they generate signals for fuel metering before the fact (the O2 sensor is reactive - it generates a feedback signal by measuring the results of combustion...)

5-90

5-90 said:

Erik -
Just an FYI, but 91-up 4.0 does not have the EGR valve, it was deleted with the changeover to ChryCo electronics. The problem of NOx was addressed with an altered cam grind. This applies to XJ and MJ.
5-90

Click to expand...

Just when ya think ya know a Jeep....

Erik :dunce:
88 Comanche

The high Nox indicates a lean burn condition which could be caused by the O2 sensor, clogged fuel injectors, improper MAP sensing, air intake leak, etc.

I would dump a bottle of fuel system cleaner in it, clean the throttle body, then replace the O2 sensor.

So the cat made it worse? Usually that will clean up the exhaust so well when new it will make the vehicle pass. I would definately change your O2 sensor. The previous responses are correct about running lean and high NOx emissions. What about an engine decarbon service? If you have carbon built up in the combustion chamber, this will cause a NOx failure too. GM top cylinder cleaner seems to work the best for removing carbon buildup. What size tires are you running? Hopefully stock size.

There is no easy answer to your problem. I would suggest doing the work yourself at first to avoid paying for the simple repairs. There is no repair cost limit for vehicles during change of ownership so having a licensed smog tech do the work is not necessary. Just make sure you check the basics first. Try the engine decarb before installing the new O2, then install the new sensor and drive it for a few hundred miles.

Bryan

while we're on the subject of failing smog checks in california my '87 failed due to high CO (carbon monoxide) readings recently. all other emissions were well within the limits, while the CO results were: @ 25 mph MAX 1.12%, mine measured 1.25% (all emissions passed at 15 mph. CO passed, but just barely).

anyone have any ideas about what I can do to remedy this? I have replaced the spark plugs and O2 sensor already, right after getting the test results. The smog tech said it likely wasn't the cat conv., since the hydrocarbon emissions were so low. The cat is only 3-4 years old.

This was done at a test-only station, so he couldn't get into likely causes/solutions for the high CO reading.

i seem to remember reading that when NO rises, then CO drops (and vice-versa). I've got very loooow NO emissions (well below average), so what could I do that would lower the CO at the expense of raising the NO a bit?

Thanks! :dunce:
Chuck

OK, bringing up a dead thread here.

I have a '94 that's not passing NOx at the 15mph test as well. I have changed out the cat and the O2 sensor so far. I am also going to change out the exhaust manifold for the APN one as the on it is cracked at the collector.

I cleaned out the throttle body when I had it off to replace the TPS. There was quite a bit of carbon build up in it. I'm assuming that I have a good deal of carbon buildup in the intake as well. What would be best to use to blow it out, GM top clyinder cleaner or Sea Foam? Do these both effectively do the same thing?