View Full Version : emissions fail- high NOx

October 20th, 2007, 18:08
i went through more than 15 pages when i searched for emissions. i didnt find anything that really answers my issue though, but that doesnt mean that i didnt miss something.

vehicle: 95 xj 4.0

situation: can not pass AZ emissions testing (done on the dyno machine) due to high NOx levels. i have gone in for 3 tests so far. 3.00 is the limit and so far i have scored in order of tests preformed, 3.44, 3.17, 3.39

my repair attempts: after first failure- new air filter, clean TB and intake hose, clean ccv lines where the oil dripps onto the air filter, seafoam treatment (total of one can in vac line, 1 can in gas tank). that dropped me from 3.44 to 3.17. i know that i have a sizeable crack as does most stock headers for the 4.0. for retest 1, i cleanned off the area arround the crack on the header and applied some copper RTV. i waited about 90 min and took it in for a retest. i failed with 3.39. when i got home i looked and my repair attempt had been stripped away.

here is a visual referance on my failure sheets.



and this is what the info packet they give me says about NOx

as far as i have been able to gather from searching, the egr was eliminated in the HO engines right?

im wondering if the crack in the header could be causeing me to run too lean? if this is a possible cause then what are some options to repair. i just got laid off on friday so $$ is a little tight right now and time is as well as the registration expires at the end of the month.

im also wondering if the cracked header caused the O2 sensor to go out causing it to run lean?

if there are any more ideas, please let me know.

help in this issue is greatly appreciated.

October 20th, 2007, 18:15
crack is probably allowing extra oxygen into the exhaust and causing the unburned fuel to flash/burn in the pipe and converter. This will produce extremely high temps in the converter which, in turn, allow high NOx readings. I would seal the crack with JB Weld type stuff lomg enough to get through emissions testing and search the "bone yards" for a replacement.

good luck!

October 20th, 2007, 22:18
When I have failed for high NOx it has been due to a bad catalytic converter. Try to seal the crack first and see if that fixes the problem. If not, I would put my money on the cat.

October 20th, 2007, 22:28
When I have failed for high NOx it has been due to a bad catalytic converter. Try to seal the crack first and see if that fixes the problem. If not, I would put my money on the cat.

Likewise. You should not have an EGR valve, as it was deleted for 1991.

October 20th, 2007, 22:43
Reading the report I am wondering how the hell they measure grams per mile of NOx?

I would suspect the O2 sensor is running too lean, but it could easily be the header leak so seal it first. It is also possible the test site equipment is a little out of calibration. Might try another site if all else fails, or if you are paying for the next inspection try a different inspection site.

The other two tests show your Cat is still working OK, though not like a new one. I would leave the cat as a last resort.

October 20th, 2007, 22:56
your states emissions does not help the diagnostics. in mass they give you oxygen and co2 so you know how the engine is running.

but is there was a leak in the header causing the o2 to run lean it would jump up the hc ppm. the high nox is from a lean condition. o2 sensor thinking the engine is lean causing high combustion temperature. if you had an egr it would be that so im at a loss.

October 23rd, 2007, 14:28

I tried the JB Weld with no luck. I cleaned the area on the header to remove all the debris, rinsed it with brake cleaner and allowed to dry. i mixed equal parts of the two tubes of JB Weld and applied to the header over and arround the crack. I allowed it to dry overnight before driving. the temps got into the mid to low 60's so there isnt the probablity of insufficient cure time.

today I went to inspect the repair, and the JB Weld had turned to ash. quite litterally. it came off with ease using a wire bush.

here is the cracked header in question
top view

bottom view

you can see the reminents of the JB Weld in between the two pipes (the light gray substance)

im thinking that cant be good for emissions. I replaced the O2 sensor at the same time i applied the JB Weld.

im thinking that im going to apply for the state program that helps with the repair costs and get myself a new header.

i would rather find the source of the problem and correct it rather than just mask it with a new cat. sure the cat is probably on its way out, but i want it taken care of right.

my question is: has anyone had a similar issue and was it corrected by repairing/replacing the header? and are there other options i can look into?

October 28th, 2007, 20:04
I wonder if a muffler shop can't weld that crack and seal it for like $20 to $30?

I don't know if it would hold up in the long run, but I would have looked to the ceramics industry for a cement - clay based patch material that can handle high kiln temperatures (like 1,200 or 1,800 F) and still have some mechanical strength and elasticity.

Organic epoxies just can't handle the exhaust manifold temperatures.

October 28th, 2007, 21:00
high nox could be caused by increased compression temperatures or lean conditions, which could also increase the temps. there are also other possibilities, but one thing i like about the HO 4.0L is that they aren't "generaly" that hard to pass emissions. hell they are one of the easiest engines to make pass in my opinion. now i didn't see you mention a tuneup, or what grade of gas you are running. i had a customer the other day who was trying all kinds of parts to get his vehicle to pass emissions. and just like you he was only failing nox and only by a bit.

i told him to do a COMPLETE tuneup (plugs, wires, cap, rotor, air filter, fuel filter) and run your tank down, and fill it up with premium gas. drive it hard on the freeway for about 1/2hour, and take it in for emissions. sure enough he called me back that same day and said he passed. now the crack in your header could be messing you up, but it would make you run rich, not lean.
well actually the o2 sensor would read lean because of the fresh air coming into the crack, and the computer would try to compensate. so your engine is actually running rich, and the o2 sensor would be running lean.

but it's not impossible. i've passed my 95 XJ 2x already with a cracked manifold.

also when you do your tuneup, make sure you use the champion plugs and good quality wires. i would actually recommend going to the dealer and buying their tuneup kit. it comes with factory plugs, wires, cap and rotor and around here, the dealer kit only cost like $5 more than the parts store setup.

hope that helps. sometimes things aren't nearly as complicated as they seem. specially with these HO 4.0L. that's why i love these engines.