Like everything it's a balancing act.
Exactly... The science behind emissions management fills text books and even entire careers, so i tried to be brief and summarize. I don't think I was either brief or successful at summarizing.
I have a good handle on the complexities of the emissions process, but this started with me just wanting to get past one last test...if I had a distributor, it would be easy to adjust timing and see if I can scrape by. With an ECU it's tougher, but still fairly easy to manage sensor readings...literally, a minute to unplug the sensor, stick the potentiomer in the circuit, replug the sensor, and start turning the dial on the POT to get the setting you want.
As you mention, it's the balancing act that makes emissions management tough. The cat can't clean CO and HC emissions without sufficient oxygen. Conversely, low amounts of oxygen are needed to clear up NOx, which as I mentioned in my original post is where my car is failing. You can't optimize for one situation and pass the other. You can't have it both ways, so you have to find the best compromise...and when it's close, a small timing adjustment can often help clear the hurdle.
I'm pretty sure my root cause is a small exhaust leak at the manifold caused by a broken stud (which is introducing the oxygen into the exhaust that makes NOx cleanup difficult). I've verified the leak by smoking the exhaust, and while I can't prove that's the cause of the fail, it typically is a major issue with NOx.
The challenge for me is that I plan to change my exhaust after the final emissions test. Anyone who's worked on the manifold, knows it's a very difficult job. To get the broken stud, and properly seat a new one, I think I'll need to pull the manifold first. That's pull it and put it back on just for the test...then pull it again right after the test. I'd rather have a fork in my eye.
My results are close enough that I'm pretty confident a 3-way cat will do the job even with the manifold leak (new cats are miles ahead of 25 year old cats with 25 year old technology in terms of efficiency and cleaning ability), but that's also a lot of engergy (and cash for a new cat) for something I don't even want to begin with. I think I could get some of the cash back, but the time to do all that work for a 15 minute test is at least equal to a poke, if not a fork, in the eye.
My easy option would be to retard timing and see if it's enough to pass...which is tough to do without a distributor. Realistically, a reprogrammed ECU just for this test isn't an option. Lying to the ECU is, however.
If I can lie to the ECU, the algorithms stay the same but results change for free and for 10 minutes of effort and a $2 POT, and after the test, it's undone just as easily ...if I can figure out what lie to tell. Tuners do this frequently to advance timing when running higher octane gas than the ECU was programmed for. I'm just trying to go the other direction for the test. The root cause exhaust leak will be addressed when i replace the entire exhaust system after the test, but I'd rather not have to do it two more times before the test if at all possible.
That's why I'm asking for ECU logic rather than emissions help. It may not work, but relative to my other options, it's worth exploring first.
Hope this makes sense.