NOX is caused by too high combustion chamber temps. It can be caused by an EGR that isn't opening when it should, a too lean condition, advanced timing..
I've cleaned and flow tested 3 sets of 3FE injectors and never seen one that had a decent flow. I had to toss at least 3-4 of them because they were hopelessly clogged. But the sets I had may have come out of engines that were out to pasture for a long time too. Anyway, like I said I still have more of them then I need if you want to try some known good injectors.