CM2350 derate 6255 only no fault codes

[greg1227]

we pulled truck out to work on some others to get them out. We just pulled it back in and customer read somewhere to look at intake , we removed the intake manifold and it is all carbond up. I know some carbon is normal but how much ? We also ran truck for a couple minutes with intake off heard a definite popping sound and it got quieter when doing a cylinder cut out test shutting off 6 . My questions now how much is normal carbon build up ? Where is all of the carbon coming from ( egr cooler leak, valve seals, rings) customer said it does not use oil or coolant abnormally .