what do you use to clean the black build-up on your rear bumper from the exhaust when running a little rich? My car soap didnt seem to work too well... maybe i just need to scrub a little more.

it may sound odd and out of this world to most people, but get a little bit of wax and rub in into the resido and then take it off, the tresido should come off

I have that too and wax didnt take it off, maybe try ammonia on a paper towel and rub it on??

how thick is the resido, you can try amonia, or if you can get any commercial strenght orange cleaner that will do the trick, but if the resido is a thin layer a wax will take it right off