NOx too high
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Joined: Dec 2005
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From: Bay Area, Calif
NOx too high
Hi everyone. So my 89 2.9L 4X4 did not pass the NOx portion of smog check. There are no vacuum leaks, no leaks in cooling system, heads are new as well as engine recently rebuilt and have only 38K miles on it. It's a wheeling toy, so I do a lot of idling, which makes me think it might be the CAT. Is there a way to rule it out or is that a shop kind of thing? What else can be causing a high NOx? O2 sensor has less than 20K on it and is a BOCH O2 sensor. Thanks everyone.
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Nox comes from high combustion temps, or a red hot cat, but I doubt that is the problem.
What was your CO number? If well below the max, you are probably looking at an EGR problem. EGR cools combustion temps, which reduces NOx.
If you were running rich and causing the cat to overheat, you might also have high HC and CO. If both of those are at the high end, run it and check the cat temp.
So, you are looking at the EGR and/or mixture, which could be sensors, throttle sensor, temp sensor, t-stat too cold, possibly other items.
If your CO number passed, you do not need a new cat.
What was your CO number? If well below the max, you are probably looking at an EGR problem. EGR cools combustion temps, which reduces NOx.
If you were running rich and causing the cat to overheat, you might also have high HC and CO. If both of those are at the high end, run it and check the cat temp.
So, you are looking at the EGR and/or mixture, which could be sensors, throttle sensor, temp sensor, t-stat too cold, possibly other items.
If your CO number passed, you do not need a new cat.
Last edited by 85e150; Jun 12, 2014 at 01:56 PM. Reason: add "new"
Laughing Gas
Joined: Mar 2011
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...unless your state requires visual inspection, in which case you could leave the cat housing on and just knock out the internals. Best of both worlds.
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You won't pass CO without a cat unless you are meeting some early high allowance regulation with a really small engine.
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