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Emissions FAIL Twice High NO

26K views 24 replies 17 participants last post by  Artinsocal 
#1 ·
Took the Ohio E-Check test today, failed. High NO levels limit:1163 Reading:1708.4
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Then I went to autozone and asked for their advice (bad idea) the told me to put in some "dry gas" to remove the moisture from the tank. Took the test again and...FAIL Reading:1902.4 HC rose and CO decreased.
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I don't know what's causing this. Could it be my bad PCV valve? My oil too high? Just replaced my oil pressure sensor. Check engine light is NOT on. '95 with a 4cyl, any ideas??
 
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#2 ·
How old is your cat? Since we don't have EGR valves ( I too have a '95 2.5L) the cats will fail sooner then vehicles with EGRs. Also have you done anything to lean your engine out? Bad CATs will raise the NOx level and bad O2 sensors will also cause high NOx levels. You may not always get a CEL with a faulty O2 sensor. Pull the "key trick" and see if you get code # 21, 51, or 52.
 
#3 ·
What year, and engine?

Basic rule of thumb for NOx:
Exhaust leaks before the cat converter
Os sensor (rich reading that might be false tells the computer to run lean).
Vacuum leaks

Anything that can cause the engine to run HOT or LEAN.

The 4.2L Jeeps re famous for leaking Pulse AIR injection systems, flooding the cat with air, disabling the NOx reduction.
The 4.2 is also famous for Vacuum lines breaking, leaking and this can cause the EGR to not function, that EGR is IMPORTANT to lowering NOx in the 4.2L

Ths 2.5LTBIs main issues are bad O2 sensors, and low fuel pressure causing a LEAN condition

The 4.0L and PFI 2.5L main issues are low fuel pressure, bad O2 sensors, and carbon on the pistons, raising compression causing higher burn temps.

ALL YJs have BAD issues with cracking exhaust manifolds, any exhaust leak before the cat renders the NOx reduction portion of the cat converter useless.
Also PCV systems plug up and raise CO, but if the line cracks and leaks there is a lean issue and Chrisler has an upgrade kit to help reduce this from happenign in the future (you might not even be able to get the normal parts, only the kit).

However, to get into more detail, we NEED your Year and Engine information.
 
#5 ·
BAH! thats what I get for looking at the pics, an not finishing the post!

YJzehe, Check for ANY exhaust leak before the Cat Converter, and if there are none, have a "Well Known" exhaust shop check your Cat, ideal CO2 should be 14.7%, you are a bit below that.
Most exhaust shops will check your cat for free, but try to find one with a good reputation, they will be less likely to just sell you a cat without checking it first.
Also, have them check the O2, its a pretty simple test, done with a digital oscilloscope, and a common procedure, if your O2 is misreporting a richer mixture, your computer leans out pulling fuel, not normally seen with your CO2 and CO readings, but it can have an affect.
 
#6 ·
I just took the test again today, after having replaced the oxygen sensor and pcv valve grommet, which has been leaking for awhile. I checked all vacume lines for cracks and replaced one the rest were good except one that was disconnected, fixed that one too. Still another fail, NO was even higher 1935. I think I'm just going to buy a new cat. In Ohio the first 3 fails are free of charge after that they cost $18. There are some exemptions, if I show a 30% change for the better and have spent more than $200, I can be waived. If I've spent $300 with no improvement I can also be waived.
 
#8 ·
Again, I highly recommend you do it yourself because an exhaust shop will want over a grand to do it, then they'll sell the old cat and make more money off of you.
If any exhaust shop is charging you that much, they are going to go out of business, Im in Cali, and Cats for YJs are around $250 installed ($350 if you are a stranger).

Its also illegal to resell a used cat here, but thats just Cali...
 
#11 ·
fourth test was taken today. Another fail after changing my cat, muffler and tail pipe. NO still the reason for failure 1,500 ppm. So far I've changed vacuum lines, ccv valve gromet, oxygen sensor, cat & muffler. I've checked the header and down pipe for cracks there was nothing. WHAT IS GOING ON!?
 
#12 ·
Have you checked for manifold leaks? These Jeeps are notorious for cracked/warped manifolds and that'll cause high NOx.

Usually someone would have piped in by now and mentioned the old sea foam and higher octane fuel trick. I recently replaced my motor so I had them run the pretest before doing the actual test. It came in 6 ppm below max, yikes! Brand new engine so Sea Foam wasn't needed but I did put some 89 octane in before the actual test and it lowered the NOx by almost 200ppm. Mines a 87 4.2L so you may get a different result.
 
#14 ·
My 92' 4.0 was high on the Nox reading, mostly due to a leaking exhaust manifold. Lean conditions caused in part by vacuum leaks will lead to high combustion chamber temps and in turn higher Nox readings. It's a snowball effect that can take out o2 sensors and cats , I would strongly suggest you seek out vaccum leaks first then work your way back, guaranteed to pass did not work for me.
 
#15 ·
So today I changed my spark plugs, because bad plugs can cause unburnt gas to be present in the exhaust? Also tried running a 1/2 can of SeaFoam through the vacuum line on the break booster, let it sit for 15 mins, started and nothing happened, no smoke no loss or gain of power. Didn't take the test today, I am still looking for what the issue could be. If all else fails I have to buy something that seems like it could help and costs more than $80 any ideas?

Have you checked for manifold leaks? These Jeeps are notorious for cracked/warped manifolds and that'll cause high NOx.
My 92' 4.0 was high on the Nox reading, mostly due to a leaking exhaust manifold. Lean conditions caused in part by vacuum leaks
I have checked the manifold and the vacuum lines
 
#16 ·
Go back and read 2xs' first post, he s a smog tech. I think you've replaced everything he suggests except he keeps repeating low fuel pressure being a cause for high NO. I don't know how to fix low fuel pressure, but maybe that might be your next direction. I used Seafoam in my gas, maybe that would help since you already have a can. It's supposed to help clean the fuel lines and injectors. Good luck, don't give up!
 
#19 ·
I am in southern CA and my 93 yj wont pass smog either. I replaced my cat plugs o2 sensor and ran seafoam in it. I am only failing nox, it needs to be at 513 but the lowest I got it too was 630....so close. This is the first I am hearing about a leak in the manifold before the o2 or cat causing high nox levels. I did notice while I was using the seafoam there was white smoke coming from my manifold I guess that is my next option. and yes I know moving is one but I cant move I have a career here that I cant leave.....in any other state my yj would pass flying colors
 
#20 ·
I had a similar issue with my jeep last spring. I replaced the cat, ia sensor, o2, tuned it up, seafoamed it verified no exhaust leaks I was still failing smog on NOx.

I talked with an old timer from the racetrack. He said high NOx is due to high temp in cylinder and said run high octain race fuel in it and take it in for the smog. I was at my last hope on getting a pass. I passes with such low numbers the smog shop thought I did a motor swap. Five failed attempts and finally a pass. It worked for me but I will not guarantee anything. Best of luck.
 
#25 ·
I also have a 1995 2.5l that failed the NO at 25 mph so this is what i did that enabled me to pass.
Replaced my Cat due to it being faulty after a year. The warranty covered all charges.
Changed the oil
Changed the air filter
Ran Lucas fuel system cleaner through a tank of gas
Ran a tank of 97 octane fuel
I can't tell you what out of all i did brought the NO down and maybe it was only the new Cat but whatever i did it worked!
Prior to doing all these things my NO @25 mph failed at 1138. The max here in Cali is 554
After doing the above it passed @25 mph with 335. That's a reduction of 803 NO ppm!
One more thing, I made sure the 2nd time around that i got the Cat really hot by driving it around for about 15 minutes. The first time i took it i only drove it from my house to the shop which is only a few minutes away. Maybe that made a difference also.
I'm happy now that i finally passed!
 
#22 ·
Check for leaks by sticking a potato in your tail pipe, and checking the exhaust while it is idling. Check carefully around the manifold, if it is leaking you can feel the exhaust by holding your hand close. Make sure not to touch anything.

The potato shouldn't be too tight so it will let some exhaust through, but it needs to be tight enough to raise the pressure.
 
#23 ·
I failed Ohio e check too when I first got my YJ. I cleaned the intake with sea foam, ran the gas tank down low, put half a can of sea foam in the tank an 10 Gal of 91 octane an got it through. I don't remember the specifics but I think my NO was 1260 an the sea foam/premium, changing plugs an wires an filling the tires up to 38 to help rolling resistance got it down to 841.
 
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