X 15 565 black oily residue

[Colter]

It will be fuel, there will most likely be a programing issue that's causing it especially if it happening at high idle as well. It might be fueling unnecessarily to heat aftertreatment which would cause the issue Cummins is trying to alleviate with their new idle recommendations.

It may also have other contributing factors in the programming as well as mechanical issues due to handicaps in the ECM, malfunctioning sensors feeding valid but incorrect information, charge air leaks, dirty intake, restricted exhaust, etc. All things that should be checked prior/during any modifications. There could be layers to the issue and other items that are currently hiding.

I would recommend pulling the current file and having it reviewed. If you need assistance with that there's load of threads on it or ask away.