First, some background...I have an '04 NBS 5.4 with a Gryphon tuner. It's got 75k miles on it.

I finally got around to giving the truck a good post-winter wash today, and noticed a lot of black soot on the rear bumper directly over the exhaust pipe.

I run good gas, pretty much Chevron all the time. I'm on the Level 1 87 tune now, but did run the 93 performance tune some during the winter. My MPGs have been pretty bad for a while, pretty much 13.5-15 all winter. Am I not burning efficiently? Is there a setting I can change in my Gryphon to fix this?

 

This is normal.

 

Oh ok thanks. Guess I always used to wash it too frequently to notice

 

Do you have the stock exhaust?

 

Yes