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Failed California Smog Test 2X

6 January 2004
Pasadena, CA
I've owned my NSX for 9 years and this is the first time it has failed smog (1991 with 197K miles). After it failed, I called my NSX mechanic, he advised to get the cats hot prior to testing. I poured some Lucas Deep Clean Fuel System Cleaner (it advertised to reduce NOX) and kept the car at about 4500 RPM for the 20 mile drive to the shop, also kept it at about 3500 rpm as I waited for the re-test. All of the emission results are well within range on both test, however, this 2nd time around the NOX emissions were even slightly higher than before.

What will likely cause the NO (PPM) emissions to exceed parameters (considering all the other emissions tested fine)? I've looked through the forums, EGR Valve Replacement, EGR cleaning, O2 sensor replacement, new cats, are all mentioned...but what should be my first step?

C02 (%)O2 (%) HC (PPM) CO(%) NO (PPM)
14.8 0.15 80 25 42 0.52 0.03 0.12 549 233 998 FAIL
14.8 0.13 62 20 37 0.61 0.03 0.1 507 194 1063 FAIL

Click on the thumbnail below for results.

Smog Test.JPG


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I talked to Robert, owner of the Clean Fuel product. He mentioned his product reduces CO2 to almost zero, but he sees about a 200 point reduction in NO (not the case in your readings, almost 0). I need to reduce NO emissions by about 500. Very nice guy, apparently there are not any local retailers in So Cali, I would need to mail him a check directly for the product. I'll order a pair of bottles, but is there anything else that might help my chances? Would suck to fail again.
My friend's '91 failed with high NO. A former NSX tech I know suggested using Chevron Techroline fuel system cleaner. Ran a bottle through the tank and on retest it passed.
Not sure if this will help but when I first got my car (1991 with 48k miles), I:

1. I ran 2 tankfuls of Chevron with Red Line SI-1. Failed.

2. Replaced spark plugs. Passed.

3. Afterwards I had the injectors cleaned by RC Engineering. 1 injector was dripping, the others had fair or good spray patterns. Variance was from 234 cc to 245 cc. Injectors all flowed at 249-251 cc afterwards, with excellent spray patterns.

At 200k miles, it might be time for an injector cleaning regardless.
I had a similar issue, the end result was the EGR was not functioning correctly. The EGR system passed the individual diagnostic tests ie the hoses held pressure the valve actuated under vacuum. But as a system it was not actuating the valve enough. On the dyno the tech could bring the car into spec by applying more vacuum to the EGR. A DIY test Daedalus posted was to attach a vacuum gage and run the tubing into the cabin. Then you can drive the car on the street at 15-25 to see the vacuum was around 6-8in. Note this all needs to be done after the car coolant system is at temperature. An indication of this is the radiator fan, know that the ECU reads a separate thermistor and if the valve is not actuating at all this could be the problem. (As well as others documented in the shop manual troubleshooting diagram)

Diagnostically I wound up replacing EGR components until it passed. I do however believe it was the EGR feedback signal that was out of spec but that was not tested.

Bottom line check the EGR and coolant sensor
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Thank you, this information has been very helpful. The closest NSX shop is about an hour drive, then I have to figure out transportation to leave the car their.

What is the best approach? Replace the EGR myself? I heard that is relatively easy to do. Or take the car to the mechanic and have them trouble shoot and fix.


I spoke to my NSX mechanic, he indicated a car came in that was failing smog, EGR, O2 sensor and Cats were all fine. Air filter was replaced, and that may have corrected the issue. His shop is quite a distance, and working out the logistics of dropping off the car and getting a rental, would be tough.

I'm just going to take the car back to the testing station with a new air filter, but on top of that I would like to clean the EGR. All the emissions were well within range except the NOX. Is there a DIY link for cleaning the EGR? I was unable to find anything.
Just google "cleaning EGR valve". There are multiple videos on youtube detailing the procedure(not NSX specific though) as well as multiple threads on automotive forums. EGR valves are relatively simple devices and the procedures in these videos and threads can be used on most if not all EGR valves.
The car is over 20 years old and they still require emisions testing? What do the muscle car guys with classic cars do?