ISX cm2350 low. power/poor. Fuel milage.

[JimT]

Yet another thought as i read through this thread again. You mentioned DEF light. When was the last time you changed the DEF filter? Also, has there been any codes in the ECM related to the DEF light? The entire aftertreatment system has to be working properly and balanced. If one part isn't functioning properly (or just one sensor reading incorrectly) it can throw off the entire system and cause all kind of power loss issues, many which aren't reported because they aren't faults, just the ECM adjusting operation to try to balance things out, so to speak. Perhaps a deeper dive with CT might be able to spot something? I'm speculating here since I've never personally used it.

Poor DEF quality could play a role. The quality of at-the-pump DEF is known to be variable. Maybe try switching to boxed DEF for a while and top it off every time you fuel up. If after a few weeks things magically get better then you'll know why.

Also, just because you replaced the DEF doser doesn't mean it fixed the problem or that the new doser was any good. Sad, but something to consider.