I finally figured out (thanks to some posts here) why I had codes after my Milltek header and Cat install (flex joint had broken and I wasn't going to pay $1500 for a factory setup). Now I'm looking for answers as to why I have high NOx in my emissions test (limit is 1.5g/hr, I'm showing 1.61 g/hr).
Both upstream and downstream ox Sensors are new (NTK brand), Purge valve has been changed, No codes, runs fine. In my research on the issue, I've found that a plugged or restricted EGR valve can cause high NOx readings, but I don't find any reference to an EGR valve in the Bentley Manual.
The NOx graph provided by the emissions station shows spikes when the throttle is backed off to maintain speed when its over ~ 30-45 mph.
For reference, Tigger is an R53, 15% underdrive pulley, JCW CAI, 2% ATI crank Pulley, Stock injectors and tune, 195k miles, now with a full Milltek Exhaust system. During my code diagnosis, it was running without the benefit of pre and post cat Ox sensors and the cam position sensor (ran quite rich) due to blown fuse #03, that's why I'm thinking the EGR valve could be restricted/choked.
Anyone with experience with this?

 

First gens don't have an EGR valve, which is why there isn't a reference in the Bentley.

 

NOx arises from high (too high) combustion temperature. At some point automakers resorted to using exhaust gas recirculation to pollute the incoming charge to reduce the heat of combustion to keep it below the NOx forming temperature.

But your car appears to pre-date the use of EGR. However another method to cut NOx formation is valve timing that has some intake and exhaust valve overlap that has a bit of residual exhaust gas flowing past the intake valves which then gets picked up and carried back into the combustion chamber by the incoming fresh charge of air.

If valve timing is questionable the valve timing method of NOx control may not be working at 100%.

Also, with the changes you have made you have of course increased power but in doing so have raised combustion temperature so the factory method for controlling NOx may not longer suffice.

At least that is a theory.

Having dealt with CA emissions testing for years with various cars -- but all stock -- all I can advise is be sure the engine is in good tune and there are no exhaust or intake leaks. And that any/all variable valve timing works as intended.

Will mention this but it may not have any application here: I had one car with high miles failed emissions with too high of HC. Long story short took the car on a spirited drive -- if you know what I mean and I think you do -- and after some several scores of miles back at the testing station for a retest engine passed with flying colors. I had made sure the engine/exhaust was fully up to temperature before the 1st test. Just lack of prolonged highway miles had the converters a bit dodgy (loaded up) and this affected their ability to process exhaust gases.

If the car is fitted with 2-way converter NOx is not processed. A 3-way converter will process NOx.

 

Thanks guys,
An "Italian Tune-up" was going to be part of my next attempt, along with an additive to try to clean the carbon off of the tops of the pistons and backs of the valves, and then a new full tank of fresh premium with no additives. Most of the additives (especially the octane boosts) contain alcohols which make the fuel burn faster, not slower and could cause the additional late burn that's causing the NOx, so I'm not going there.
Also good to know there's no EGR to clean (drat).

 

I have found Techron can make a difference. Get a bottle from the auto parts store. Use according to directions. My experience is just filling up one car's fuel tank with Chevron Supreme 91 (E10) with Techron and by the time half a tank of fuel was used the engine was running better. Low speed throttle response was much better. And yet it didn't feel like it was bad before it got better.

If the engine appears to benefit from the Techron you can do a 2nd bottle.

Now that was with one car. Naturally aspirated engine. High miles (>250K) but engine was in good tune and good condition. My 2nd car a turbo charged engine with around 125K miles -- also in good tune and good condition -- didn't manifest any signs of a benefit from the Techron.

I switched to Chevron from Shell because the Shell station upped its prices while the across the street Chevron station went the other way.

Also it didn't come up but be sure the engine air filter is good. Embarrassed to admit it but I let the engine air filter go too long on not one car but two cars. One car I just lost track of time and miles. But when I got around to changing the filter it was filthy.

Second car only had around 27K miles on it. Was about 33 months old. Had it in for an oil/filter service. Asked the engine air filter and cabin air filters be changed. Both were filthy. Good grief. I didn't live in a real dusty/dirty area. But the engine, a 6.2L super charged engine making 707hp (in a Dodge Hellcat) really sucked in the air. On the way home with a new engine air filter the engine was running noticeably better.

Just bringing up the air filter. With a turbo charged/super charged engine it uses lots more air than a naturally aspirated engine. And the air filter of course has to filter all the air. You don't want to leave anything on the table trying to get the engine to pass emissions.