My V6 with 154k miles failed just like it two years ago. NO failed at 605 out of the allowed 424. It's not the EGR, I replaced that for this same reason two years ago, it failed again then passed randomly with great measurements...

I'm thinking it could be my K&N inputing too much air and creating a lean condition or that my cat is not getting hot enough, how long do you guys run your cars before a test?

 

NOx is almost always EGR. If it was a lean or rich issue you would see high CO. I don't know all the details on the V6 EGR system, but maybe your valve is getting clogged up, or the pipe is clogged. Try taking the valve off and cleaning it out a bit.

 

Whoooaaa....

Lean conditions cause high NOx, low HC and CO. Rich conditions cause high HC and CO, low NOx.

But believe me, its not the "K&N inputing too much air" and causing a lean condition. Air consumption is affected in a very minor way by changing to a low pressure drop air filter, and primarily at full throttle/high RPM, not at the conditions they use in the 15mph emissions test. The minor change in air flow is definitely nothing the O2 sensors can't correct in closed loop, if it happens at all. And the filter doesn't "input" air... its actually a resistance to air flow.

Definitely worth establishing that you have EGR flow, but do not rule out a lean condition. Also check your engine coolant temp. High combustion temperatures also cause NOx formation.

 

Injuneer:
Not meaning to hijack the thread but how does spark advance affect NOx during EGR? From other threads, ISTR the ECU adds additional advance when EGR is activated. What happens if the EGR system is inoperative but the ECU is still adding advance?

 

I don't know because I stopped using my stock PCM 9 years ago.

 

Darn. I was hoping you could provide some insight into the effect of spark advance on NOx emiisions.

 

Advancing the spark at low rpm/high load increases combustion chamber temps and increases the chances of detonation. The purpose of the EGR is to reduce combustion chamber temps under those conditions. Logically, you could use a bit more spark advance with EGR working. More advance (as long as it doesn't lead to detonation) will increase fuel economy at cruise conditions. The EGR is actually reducing the usable volume of the combustion chamber by filling it partially with an inert substance.... sort of like a form of displacement on demand. I suspect it may also improve fuel mileage very slightly.

What I don't know is how the stock programming handles the timing, fuel, etc. when EGR is operating. If I had kept using the stock PCM, I might have figured it out. I guess I could look back at all the data logs people have sent me, and try and correlate the changes in advance to the EGR duty cycle.... but I don't seem to have the time to do that kind of stuff any more, not working a 60-hour week.

 

Injuneer:

I was only wondering if a car was borderline inspection failure on NOx, would pulling a few degrees of advance be helpful. Sounds like it would be if it lowers combustion chamber temps a little.
In the meantime, thanks for the time, effort and valuable information you contribute into the Camaro forums. It'll be spring in a few months and you'll get to enjoy the fruits of your personal 4 wheel labors. Hope you lower your ETs AND increase the big end numbers.

Have a Happy New Year