Inspecting a few things on the rig this morning and noticed a very fine black deposit on in the coolant expansion tank. It is on the upper plastic walls of the tanks. There is not much of it just enough to discolor the plastic. There does not seem to be oil in the coolant, it looks much more like exhaust gas has bubbled through some where. Before I panic is there other sources I need to consider? I did have a water pump failure last summer, but never saw the temp gauge read higher than normal and replaced it right away when I saw it leaking. I am new here and have tried searches and come up dry, I may not be using the correct search terms? ANY thoughts, ideas would be appreciated. On a related note if it is a head gasket, at 300K miles should I look at a valve job? I've heard the top end is good for 500K?