BLACK RESIDUE IN COOLANT RESERVOIR

[TRUCKING365]

Hello everyone,

I am new to the forum and trucking in general. I have a 2015 Freighliner Cascadia with a dd15 in it. I am currently doing maintenance on the truck and noticed I have some sort of black buildup in the coolant reservoir. I did a couple of google searches and came across this forum. Currently seeking help to determine what the problem might be. Temperature in engine is fine, does not seem to be consuming coolant either. Someone suggested EGR cooler going bad; however, upon checking the hot pipes, they seem to be OK and dry. Any ideas or suggestions as to what it might be? Another thing I came across during my search was the option of injector caps and o-rings gone bad causing diesel fuel to leak into the coolant. I do not know enough about injector caps in order to determine it being the cause. Thoughts? I thank everyone in advance for the help.