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Old 11-11-2010, 09:16 PM   #1
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Default Smog Question.
What would cause a 4cyl Jeep to pass HC, and CO with flying colors, but fail NO at 15mph but not at 25mph. Any ideas? It has a brand new cat (smog legal) that is clearly working for the most part. Help
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Old 11-11-2010, 09:30 PM   #2
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I would check the EGR system. Faulty EGR can cause the NOX to be high. Also if you have too much timing advance can cause elevated NOX.

Bob
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Old 11-11-2010, 09:38 PM   #3
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I would check the EGR system. Faulty EGR can cause the NOX to be high. Also if you have too much timing advance can cause elevated NOX.

Bob
Stock tune. I'm thinking EGR.
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Old 11-11-2010, 09:42 PM   #4
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Quote:
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What would cause a 4cyl Jeep to pass HC, and CO with flying colors, but fail NO at 15mph but not at 25mph. Any ideas? It has a brand new cat (smog legal) that is clearly working for the most part. Help
Assuming the engine is otherwise workign to spec, the system is correctly metering emissions, that is clearly a failure of the reduction catalyst stage. I don't think there's any real break in period for a cat and I can't imagine it was a failure in manufacturing. Was the cat hot? It's always best to get a system good and warm before testing for emissions.

NO is (to my knowledge) only formed from high combustion tempratures. This can happen for a variety of reasons. The top two that come to mind are a failing EGR (which lowers charge temps) or if there's a ton of carbon buildup in the cylinders that can cause some high residiual heat.
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Old 11-11-2010, 09:49 PM   #5
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The smog tech thought it might be EGR too. Besides the EGR valve, what else is the system should be checked/replaced?
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Old 11-12-2010, 02:35 AM   #6
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You may want to pull the plugs and see if the engine is running too lean. Too lean condition will also elevate NOX because the combustion temps will be higher.

Bob
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Old 11-12-2010, 03:04 AM   #7
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You may want to pull the plugs and see if the engine is running too lean. Too lean condition will also elevate NOX because the combustion temps will be higher.

Bob
Lean = higher temps = NO
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Old 11-12-2010, 08:29 AM   #8
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What year Jeep?
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Old 11-12-2010, 08:56 AM   #9
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Hey everyone,

My Jeep is the one in question here. I appreciate all the feedback. It's a 1998 4cyl 2.5L with manual transmission (AX-5).

The two popular opinions in here are that it is either running lean or the EGR is failing, both of which were mentioned as possibilities by the smog tech. He indicated that CO2 and O2 levels gave further encouragement that it was running lean.

I'll post up a scan of the results in a little bit.
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Old 11-12-2010, 09:02 AM   #10
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Here are the results. If someone could provide some guidance as to how correct the air/fuel mixture (if that is indeed the problem), that'd be great.

I'll look on the Jeep forums and try to find a guide to replace the EGR.

Also, another relevant factor, I'm running 31's. Would running smaller tires help pass smog as the engine wouldn't be working as hard?

Thanks again.
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