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What causes high NOx on a smog test

hidesertwheelin

hidesertwheelin

1/2 ton status
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Nov 13, 2004
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Location
Hesperia, CA
So my Suburban won't pass smog. The only test I am failing is the NOx test at @ 25mph. I am emitting 1100 ppm and I need to be under 950 ppm. I have used seafoam, and still did not pass. There is an exhaust leak before the catalytic converter. Do I need a new cat? Any other causes? ERG valve maybe?
 
tbi needs 195 to run right, and make sure your egr is working. ports like to plug up.
 
I'm going through the same thing right now myself. High NOx is caused from high combustion temps usually. Make sure the EGR system is functioning properly (i just finished checking mine if you need any advice) since the EGR lowers cylinder temps this helps reduce the NOx reading. If that checks out ok then a new CAT should do the trick (which is what i'm about to do now, don't forget that it MUST be a California approved CAT as of 1-1-09).
 
I agree with the above, sounds like a plugged EGR system. although you may be on track with the cat as well. an exhaust leak could just be an exhaust leak, but it could also be a clogged cat causing excessive backpressure which caused the leak.

EGR is easy to check, take it off, visually check for carbon build up (there will definatly be some, just a matter of how much). ive filled them with carb cleaner before to clean em out, then go at em with a pipe cleaner to get all the crap out. to test the diaphram, push the diaphram open using even pressure with your fingers on both sides of center. then with it fully depressed (open) put a free finger, or thumb, over the vacuum port, sealing it. with that hole covered, release the diaphram with your fingertips. it should stay in place. if not, it needs to be replaced. if that works, the next step is to check the egr solenoid, its the small black block that is connected to the egr by the vacuum line.

My camaro is doing the same thing. High HC, Low CO, High NOx, Low CO2, turns out my egr solenoid was bad, but from running that way for so long, i wound up also fouling my cat, which was shown by it literally blowing free from the exhaust system under hard acceleration. guess you could say there was a little back pressure. Ill probably have to replace both.
 
K5dreamer said:
EGR is easy to check, take it off, visually check for carbon build up (there will definatly be some, just a matter of how much). ive filled them with carb cleaner before to clean em out, then go at em with a pipe cleaner to get all the crap out. to test the diaphram, push the diaphram open using even pressure with your fingers on both sides of center. then with it fully depressed (open) put a free finger, or thumb, over the vacuum port, sealing it. with that hole covered, release the diaphram with your fingertips. it should stay in place. if not, it needs to be replaced.
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Well the diaphram fell back down, but so did the one on the brand new one I bought :confused:
 
who did you have smog it. i live in hesperia and mine done at moms hes a good and knowledgeable guy and reasonable price to
 
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