Most likely it is heat. Not instantaneous but from driving for long periods of time with EGT's around 800 degrees. You can spray the area with PAM cooking spray and then you can wipe the black soot right off. Mothers polish works very well at shining them up, and there is a product call "blue buster" for cleaning chrome exhaust pipes that will take the blue out if need be.

 

i can't explain it. maybe just a reaction between the exhaust particulates and the bumper's finish. could be heat. i guess i've never really paid attention to the tail pipe placement on a 3rd gen. i'd think it'd have to be pretty much dumping exhaust right on the bumper all the time to discolor it though.

 

Here is a picture with the stock exhaust placement.

 

+1, I've even taken pics of my bumper/back quarter panel on trips to see how sooted up it can get...

 

Poser
I so want to install a 195lph fuel pump with a 2 gallon fuel cell with 2 windshield wiper squirters under the rear of your truck. Load that full of 50/50 H2O & dish soap. Hell you'd really be a true version of "Spy Hunter" then. Smoke and a slippery slick behind you LOL.




Seriously though if you really want to remove soot that is "baked" on go to Les Schwab and buy some of their metal polishes. They come in containers and you tear little chunks off to polish with. They all work awesome depending on the levels of oxidation or tarnish. Also invest in a Flitz ball as well.