Hello I'm new to the ecodiesle community. Today I noticed that there is what looks like black soot on the walls of the coolant tank. Anyone have any ideas of what is the cause? I have a 2015 Laramie with 62,000 miles on it . Bought it used from the dealer black Friday weekend 2018. I hear it might be a egr cooler issue. Truck has been running great no codes or other issues. Mpg are mid 18's city driving.