2012 ISX 12 CM2250 - rough under throttle

[JimT]

not familiar with the isx12, but I have a 2012 pete with the isx15 and I know that these aftertreatment systems are notorious for bad NOx sensors. Sometimes supplying valid but wrong values, most notably the 2nd sensor at the end of the exhaust, after the SCR can. The other common issue is DEF buildup inside the decomp tube, the so called DEF/SCR volcano. Either issue could cause the system to go into SCR warmup mode. You could also have clogging in the SCR can if soot or other particulate somehow got passed the DPF. If you haven't yet I would check out the NOx sensors, SCR can, decomp tube and DEF doser valve.

The only times I ever had any smoke come out my exhaust pipe with an intact aftertreatment system was:

1) when the DPF/DOC were no good and the injected fuel was not catalyzed properly, coated the DPF, caused enough restriction to trigger a regen and slowly smoldered as the system attempted to active regen (parked regen usually worked properly though). This was grayish with a heavy fuel smell.

2) when my EGR valve died. The spring inside the valve broke. I could push it open with my finger using little effort and it wouldn't spring closed. That resulted in black smoke when under a load/high boost.