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Unread 01-14-2011, 03:21 PM   #1
chrisn82
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1995 XJ Cherokee 
 
Join Date: Mar 2009
Location: Dallas, TX
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High NOx...again...

So...it's that time of year again...emissions testing for my ole Jeep. She let me down...again...

1995 Jeep Cherokee SE, 2WD, 5-spd, 4.0 I6......222k miles and runs like a champ, minus the NOx problem...no overheating issues

Last year I failed miserably. After that I did a basic tune up and oil change...plugs, wires, rotor & cap, fuel filter, t-stat. I think I also replaced the coolant while I was at it. Then I did a complete sea foam job (gas tank, vacuum line, crank case, and sprayed into intake). The sea foam helped the HC a lot and on my next test CO, CO2, and HC were lower...but NOx was still bad. I replaced the cat with the Magnaflow direct fit I ordered through 4 wheel parts. I talked to a Magnaflow tech this afternoon and he confirmed that it is a 3-way converter. After the new cat the Jeep passed well within specs.

Yesterday evening I took it in for the yearly check up and failed on NOx. Here's the readings:

Pollutant..........High Speed.............Low Speed
................LIMIT....CURRENT....LIMIT....CURRE NT
HC(PPM)......151.........30...........155......... 25
CO(%)........1.68.......0.18..........1.31.......0 .23
CO2(%).......NA.........15.0..........NA.........1 5.0
O2(%).........NA.........0.2...........NA......... .0.1
NOx(ppm)....956.......1391.........1046.......1585
DILLUTION(%)...>6....15.2...........>6........15.2


I checked this with the calculator at:
http://www.smogsite.com/calculators.html#lambda

It shows a slightly rich condition. I was expecting it to read a little lean based off of my reading.


I do not recall if I changed the O2 sensor last year. I'll probably test it tomorrow when I've got a friend to hit the throttle for me. I've heard some conflicting info on whether or not the O2 sensor will actually make a difference though. I haven't sorted out yet whether its just checking the condition of the cat or if its controlling the fuel injectors at all...anybody with a definitive answer on this? I checked the computer codes by flipping the ignition 3 times......all I get is "55" so nothing is showing up there.

I have not checked the fuel rail pressure either and I haven't checked the timing. It's possible that the chain has stretched, I'm not sure that it has ever been replaced...but it seems to run great and I don't want to go digging into all that without reason.

Another thing that may have cause a problem, and I'm kicking myself in the butt for....I don't think the cat was fully up to heat when the test was run. I stopped in on my way home from work because the place was open and not busy, I was only driving on it for about 10 minutes before and it wasn't highway driving. Plus...its like 30 degrees F out and the tech let it set and cool down for a good 20 minutes before starting it up to run the tests. Would the cold cat show abnormally high NOx, even though the CO , CO2 and HC were all seemingly low?

Any info you guys can help with would be great.

Thanks!

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Unread 01-14-2011, 03:28 PM   #2
mgramann
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Cat must be up to temp to work properly. The engine also must be up to temp to ensure it is in closed loop, and not bypassing the oxygen sensor. When a vehicle is in open loop it tends to run rich.

The first thing I would do is make sure the cat is nice and warm and try to re-test.
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Unread 01-14-2011, 03:32 PM   #3
chrisn82
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Quote:
Originally Posted by mgramann View Post
Cat must be up to temp to work properly. The engine also must be up to temp to ensure it is in closed loop, and not bypassing the oxygen sensor. When a vehicle is in open loop it tends to run rich.

The first thing I would do is make sure the cat is nice and warm and try to re-test.


Yeah....that one is nagging at me. I should know better too! I get the impression that the guys at this shop aren't too cool though and I dunno how many times they'll let me re-test. I'd hate to just go out and run it up and down the highway for a while to heat it up then bring it back in to test and have it fail again and find out that was my only shot at a free re-test....so I'd like to look into other possible causes before taking it back.
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Unread 07-20-2011, 01:36 PM   #4
moers130015
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i have a 89 but my nox was extremly out of it its like 3000 needs to be 1200 any idea what it could be 89 jeep 4.0 4cy 2wd automatic
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Unread 07-20-2011, 09:44 PM   #5
AZ Jeff
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High NOX is caused by higher than normal burn temps, which in turn are a sign of a lean mixture.
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Unread 07-20-2011, 10:10 PM   #6
96jeepchero96
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It's your catalytic converter. When I bought my XJ the guy I bought it from hollowed it out and didn't say anything and my noxs was extremely high. 500 bucks later it passed with flying colors
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Unread 07-20-2011, 11:11 PM   #7
Reptile
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The cat has to be very warm or it won't do its job. But still running the engine 10 min should be okay.

Check your EGR valve. It's supposed to open when the engine gets warm to let some exhaust back into the combustion chamber, which in turn cools it down and restrict the amount of NOx being produced. A stuck EGR can create more NOx than the CAT alone can handle.

I don't think you're running too rich. Your CO and HC are looking pretty good.
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Unread 07-21-2011, 11:11 AM   #8
moers130015
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i just looked at it there is some plastic thingy in front of my egr valve but i took my valve off it was filthy so i cleaned it and put it back on it ran a little better but i found out the plastic thingy is broken so im not geting any vacume anyidea what it is?
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Unread 07-21-2011, 06:50 PM   #9
Reptile
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Probably your EGR solenoid, but you can post a pic. The valve is vaccum operated. It should open when engine is warm.
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Unread 07-21-2011, 07:16 PM   #10
98muddyjeep
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try the seafoam again and run it for about 50 miles before the reteast,We used to sell fuel injector cleaner to all customers who failed emissions back when PA went to the very strict dyno emission tests.95 percent of the people who failed and spent the 60 bucks for the injector cleaner drove for 50 miles passed with flying colors,dont know why or how but I have seen it work and was impressed.so i would give it a shot.
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