Failed smog test twice due to NOx - JeepForum.com
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post #1 of 42 Old 05-21-2019, 11:25 AM Thread Starter
Hulkjeepyj
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Failed smog test twice due to NOx

So for several years I've had a crack on my exhaust manifold that has gotten bigger over the years. Last month I failed the 25/25 NOx test by 300 ppm. Over the weekend I replaced the exhaust manifold and the jeep runs a lot better now! However it failed the second smog test for the same reason, this time the NOx is beyond the max allowed by 200 ppm. It decreased a little bit but still failing. I'm thinking it's my cat converter...I replaced it 8 years ago but I'm assuming that crack in the manifold may have busted it? Any thoughts?

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post #2 of 42 Old 05-21-2019, 05:01 PM
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Quote:
Originally Posted by Hulkjeepyj View Post
So for several years I've had a crack on my exhaust manifold that has gotten bigger over the years. Last month I failed the 25/25 NOx test by 300 ppm. Over the weekend I replaced the exhaust manifold and the jeep runs a lot better now! However it failed the second smog test for the same reason, this time the NOx is beyond the max allowed by 200 ppm. It decreased a little bit but still failing. I'm thinking it's my cat converter...I replaced it 8 years ago but I'm assuming that crack in the manifold may have busted it? Any thoughts?
Just went through this with my sons XJ. Replaced the exhaust manifold and it got better but did not pass. Had to replace the CAT and once I did, the SMOG tech I go to for all my stuff said it's too clean now. He suspects they will ask him about it because it went from very bad to new Honda numbers in less than 4 hours.

Replace the CAT and all should be fine. What happens is when the exhaust manifold has a leak, the O2 sensor runs the engine rich. All that extra fuel stresses the CAT and eventually wears it out.
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post #3 of 42 Old 05-21-2019, 05:31 PM
jeepster93
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High NOX is usually a bad cat.
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post #4 of 42 Old 05-21-2019, 06:36 PM
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Just curious if you've tried new filters (air, fuel) and spark plugs. Checked timing. A tune up goes a long way.

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post #5 of 42 Old 05-21-2019, 07:40 PM Thread Starter
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I did a full tune up myself last year (spark plugs, cap and rotor, fuel filter, valve gasket, etc.
I went to a mechanic today and he gave me some free advice to replace the O2 sensor and run a tank of high octane gas mixed with fuel system cleaner so I'm trying that out right now. I mentioned the cat converter to him but he seemed confident it wasn't it. I might try testing the cat converter at home using an infrared temp sensor just to be safe.
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post #6 of 42 Old 05-21-2019, 08:06 PM
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I had a badly cracked header (we all have, I'm sure) for years, and it always passed smog with super low numbers. Lower than many new cars. Fixing the crack didn't change the numbers much, if at all. It's probably your cat or a sensor.
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post #7 of 42 Old 05-21-2019, 08:11 PM Thread Starter
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I had a badly cracked header (we all have, I'm sure) for years, and it always passed smog with super low numbers. Lower than many new cars. Fixing the crack didn't change the numbers much, if at all. It's probably your cat or a sensor.
My gut feeling is that it's a bad cat. I changed out my O2 sensor and it was burnt to hell but these YJ's can run decent without an O2 sensor since the computer runs at a default setting if it doesn't get an O2 reading.
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post #8 of 42 Old 05-21-2019, 08:30 PM Thread Starter
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Also when I did my exhaust manifold replacement, I noticed that this rubber tube coming from the air intake to the PCV valve is busted. I was thinking on replacing this as well or tape it up. Could this be causing a NOx issue?
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post #9 of 42 Old 05-22-2019, 09:16 AM
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If you got 8 years out of an aftermarket CAT, that's pretty good. They are not nearly as robust as the originals were. Just replace it and be good for another 8.

I didn't know GA had smog testing. And that surprises me. Is that just around Atlanta?

I don't think the CCV hack is doing anything negative. Around here they would probably fail you for a visual on that. I fail you for a visual on the green tape, but I'm sure it is still collecting vapors just fine.

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post #10 of 42 Old 05-22-2019, 11:16 AM Thread Starter
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If you got 8 years out of an aftermarket CAT, that's pretty good. They are not nearly as robust as the originals were. Just replace it and be good for another 8.

I didn't know GA had smog testing. And that surprises me. Is that just around Atlanta?

I don't think the CCV hack is doing anything negative. Around here they would probably fail you for a visual on that. I fail you for a visual on the green tape, but I'm sure it is still collecting vapors just fine. [IMG class=inlineimg]/forum/images/JeepForum_2016/smilies/tango_face_wink.png[/IMG]
Haha good to know. They only require smog testing in Atlanta and its suburbs. I changed out the O2 sensor and put in 93 octane with a can of seafoam so I might try this first then buy a new cat if I fail the test again. The jeep runs like brand new so I'd hate to drop money on a cat converter if I don't need to test the jeep anymore after this year.
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post #11 of 42 Old 05-22-2019, 06:53 PM
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NOx is formed when combustion temps are too high. Slowing burn down can help by causing more combustion to occur after piston reaches TDC. Don't add alcohol or a light hydrocarbon cleaning solution. 4-6 Ozs of diesel fuel or Marvel Mystery Oil per gallon of gasoline can actually make a slight difference. It can be hard to tell how much additive you need so the better answer is to get the new converter. Aftermarket cat is only required to live for 20k miles or a couple of years.
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post #12 of 42 Old 05-23-2019, 06:15 AM Thread Starter
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NOx is formed when combustion temps are too high. Slowing burn down can help by causing more combustion to occur after piston reaches TDC. Don't add alcohol or a light hydrocarbon cleaning solution. 4-6 Ozs of diesel fuel or Marvel Mystery Oil per gallon of gasoline can actually make a slight difference. It can be hard to tell how much additive you need so the better answer is to get the new converter. Aftermarket cat is only required to live for 20k miles or a couple of years.
Would a Magnaflow Direct Fit be sufficient or do people recommend another type for a Jeep YJ?
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post #13 of 42 Old 05-23-2019, 06:43 AM
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Originally Posted by 1project2many View Post
NOx is formed when combustion temps are too high. Slowing burn down can help by causing more combustion to occur after piston reaches TDC. Don't add alcohol or a light hydrocarbon cleaning solution. 4-6 Ozs of diesel fuel or Marvel Mystery Oil per gallon of gasoline can actually make a slight difference. It can be hard to tell how much additive you need so the better answer is to get the new converter. Aftermarket cat is only required to live for 20k miles or a couple of years.
NOx is also formed by preignition and extra air intake, so that PCV leak or any little tweak on distributor advance can affect it. I've seen California shops 'de-tune' classic cars to make them pass NOx standards, it really POs owners. Intake gasket leaks can affect it as well, but we won't go there, cross our fingers, since you were just messing with those bolts when you replaced the exhaust manifold. I'd spend some pennies instead of using tape to buy a new hose and/or PCV valve if it's broken so your Jeep doesn't fail on the visual, and hit it with a timing light to check it if you have that kind of experience.

Measure it with a micrometer, mark it with a crayon, cut it with a hatchet.
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post #14 of 42 Old 05-23-2019, 09:05 AM
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NOx is also formed by preignition and extra air intake, so that PCV leak or any little tweak on distributor advance can affect it. I've seen California shops 'de-tune' classic cars to make them pass NOx standards, it really POs owners. Intake gasket leaks can affect it as well, but we won't go there, cross our fingers, since you were just messing with those bolts when you replaced the exhaust manifold. I'd spend some pennies instead of using tape to buy a new hose and/or PCV valve if it's broken so your Jeep doesn't fail on the visual, and hit it with a timing light to check it if you have that kind of experience.
I don't know the 2.5L very well, but I believe that is a CCV, not a PCV, so it won't ever be a vacuum leak. All good points made other than that little one.

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post #15 of 42 Old 05-23-2019, 05:30 PM Thread Starter
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So I ran the temp test on my cat converter and I'm getting about 300-350 degrees inlet and about 400-450 degrees outlet. Based on those numbers I think the cat converter is running fine. Might need to replace it anyway if I fail the emissions test for the 3rd time.
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