Originally Posted by
rybo
Intake is more of a "demand" than a supply system. The engine draws the amount of air it needs. If you were adding forced induction of some type I could get behind it being a problem, but the ECU in the car should be able to adjust mixture to cope with any range of intake options, even up to removing it entirely. I'm not thinking it's the culprit.
The other system to consider is the EGR system - if there is a problem there it could cause high NOX, but I would suspect there would be a code in the ECU if that was a problem.
NOX is a result of high cylinder / combustion temperatures. There are a couple of ways that this can happen, and it is the primary reason we've seen the demise of high compression / high performance engines. So, a high compression ratio makes more NOX than a low compression ratio. You can get additional compression by there being a bunch of gunk in the engine taking up valuable cylinder space. Engine overheating can cause it because cylinder temperatures get too high. Too much exhaust gas from EGR can cause it because that air is "pre-heated" -
I would start with good fuel, and some cleaning and a nice drive with some "IT tune up" and take it back. If your number was just a LITTLE outside of the acceptable range then this might clear it up. If it was a lot, then you have to diagnose further, but the most likely cause is a dead catalytic converter.