rrich

Several half truths on this thread.

READ THIS!
http://www.catalyticconverter.org/ne...rter-&News=216

There are several easy tricks to use if the NOx is close to what it should be.

BUT - we need the numbers to help

DFlintstone

Does anyone concur with too lean resulting in a high Nox reading? (my smog license expired after they went from electric to gas).

I'm not really buyen the CCV angle. 2.4mm (or 2.6) orifice, ..designed "vacuum leak" (to just suck...)

So, an exhaust manifold crack, giving the 02 sensor more 02 would tell it what? The puter' would think it needs more fuel. That oxygen needed hydrocarbon, (fuel) to combine with. Meaning I think air to the 02 will make inrichen the mix if it scenes extra 02 from a crack.

Nothing against giving a cooler plug a shot to get through...just wrong though. Besides that it may well not work, your problem is elsewhere. (again, I might try another shop)(I saw on the internet one of those guys is after your $. )
....
Vacuum leaks suck. I go around with short little shots of starting fluid, if you hit one the sound of the engine will change. Small little shots! You don't want that stuff to accumulate. It's a fire hazard, but It evaporates right away. Also with a tube, or a section of hose you can listen for it. Don't forget the lower O rings on the injectors need to seal as well. (uppers would leak fuel).
The manifold bolts have a habit of loosening, especially that rear one. On anything like that you never want to tighten only one. It can warp/bend, even crack things. If I found that any that where easy to reach where loose, I'd pull the air cleaner and tighten them all, starting in the middle and working out towards the ends. I go over about three times. It's a bear of a spot to get a torque wrench on them all. Just don't ape on it. If its firm and not turning, no point in going on to break it, a REAL *****. The rearmost bolt underneath takes a little doing, but it can be done with the right extension. For that very back one by the firewall I use way long extensions (18"), with a swivel at the socket. Anyway, that's a good thing to check on any old Jeep.

DFlintstone

After reading (some) of Rrich's very informative link, I remembered I wanted to mention checking the fuel pressure. No matter what anything else tries to to, if being lean is causing high NOx, and the pressure ain't there, there you are!