High NO reading on Smog Test? - Fuelly Forums

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Old 01-10-2012, 11:02 PM   #1
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High NO reading on Smog Test?

Does anyone know what causes a high NO reading on a CA Smog test?

I took my car (2000 Acura Integra LS)in for its' preregistration renewal smog test last weekend. I had to go to a Test Only station per the renewal notice. When the car was tested it came out just a few parts per million below the nearly 500 that were acceptable for the 15 MPH NO reading. The 25 MPH readings were something around 17 PPM out of an allowable 500 or so. The engine RPM was within 50 RPM or so due to gearing. What I'd like to know is if anyone knows what causes a high NO reading and if there is something I can do or replace to decrease the near failing values. The Smog Tech couldn't or wouldn't tell me anything answering only "I just test them, I don't know why they come out the way that they do, all I care about is seeing that the car passed."

I'd like to repair the engine or emissions control system to make it work better, burn cleaner, and reduce the emissions but don't have any idea where to start. Any suggestions or help would be greatly appreciated.
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Old 01-12-2012, 03:14 AM   #2
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NOx emissions are related to high combustion temps. EGR is used to cool the combustion and reduce NOx. Is your EGR in good working condition?
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Old 01-18-2012, 10:29 PM   #3
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Good Question BDC, I appreciate the input and suggestion, I may have a partially obstructed or sticky EGR valve. I'll have my mechanic check that out at my next service. Thank you.
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Old 01-26-2012, 02:52 AM   #4
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If you have a cone filter without a proper cold air box and snorkel, you'll have higher intake temperature and consequently higher nox readings while on a testing dyno. I had to buy a stock intake for my Miata when I moved to Texas, in order to pass emissions - it brought nox levels down by several hundred ppm over the k&n filter that came with the car.
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Old 01-29-2012, 04:13 AM   #5
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NOx is caused by high combustion temps. The combustion temp can be affected by air/fuel mixture being too lean or by ignition timing being too far advanced. EGR also lowers combustion temps by introducing inert gas, reducing the available oxygen.

You can tell if the car is running too lean by looking at the report. It's a little more than I want to get into on this post but any qualified smog technician should be able to interpret the report. Your best bet would be to take it to a shop that is allowed to do the repairs (test only is not) and pay for a diagnostic. The months of training and years of experience are well worth the diagnostic fee which is probably about the same cost as a misdiagnosed part replacement. Just check the reputation/certification/license status of the shop as there are too many in this business who are out to make a quick buck.

Good luck
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Old 02-01-2012, 01:44 AM   #6
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Rem83 and ChrisM, thank you both for your input. The air box and intake are stock, I bought the car new and by CA Smog law can't legally add a cold air intake. The air filter may be dirty, I can change that.

As for the mixture, the rest of the numbers are right at the bottom of the measurable levels so I don't think that is it since a lean mixture should add other values to the report. The Test Only Tech was far from helpful and basically just said, "I only test them, I have no idea what the results mean." I know that a test only station can't repair, but would have appreciated a direction to go in. I wouldn't have used the test only station if it wasn't required on the renewal notice. I'll get a better qualified tech to look at the results and see if they can diagnose it that way or run a diagnostic series on it.

It is almost due for service anyway, one of the valves is rattling slightly and the rocker cover gasket needs to be replaced along with a trans fluid flush and filter change, I'll just add smog diagnostic to the list.
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