Black exhaust soot. Checked plugs on the "sooty" side and found both plugs on the same coil pack to be corroded looking. I changed coil pack with another and drove for a few 1000 miles and plugs at the new location, suspect coil pack, starting to look corroded now as well. Plugs at old coil pack location still look fine. Hmmmmm, lol. Looks like a new pack it is. At least it wasnt valves or something else harder to take care of.