2012 ISX 12 CM2250 - rough under throttle

[Mikanic]

I have never dealt much with ISX 12, but I went and got a peak on quickserve and the jakes seemed very different to the bigger engines. Scroll to group 20 in the service manual, there is some info there that could help.

I went back and read your post again. If the smoke as bad as you say I feel like a bad injector would not be your issue, but if it was a cut out test would of probably help you isolate this. I personally feel like if you had a leaky injector and all the timing and compression was equal you would not stumble much or have a rough running engine.

I think you may be in the right direction with unpluging the EGR valve. You said you though it was the turbo at one point because of VGT position. Monitor heavily and graph it so you can look at it after your road test, your sensors that control turbo behavior. Like I said before the exhaust gas pressure sensor I feel like is often overlooked, check your ports and tubing on it. EGP should behave similar to MAP, but always higher. If your EGP sensors reading is off and there is excess backpressure, you could still have incomplete combustion creating lots of smoke and probably bad enough to create a stumble.

Something else I would try is running it straight pipe with DPF unpluged for a bit, until it faults up. Take a good look at the exhaust gas. Is it smoking at start up? Does the smoke get worst at higher rpm? Unplug EGR temp sensor, then is it still smoking? Does the smoke eventually clear up? Do your cut out test and see if it changes more on some cylinders?