exaughst bild up on the rear bumper
 

is this a signe of a problem natural or just from idiling too much ? or lack or washing :-p

Get an exhaust that exits after the rear bumper. My Imprezas rear bumper gets blackened by exhaust and short flames, but my RX-7 pops, backfires and throws flames all the time and my bumper is fine since my exhaust exits right after the bumper rather than at the bumper.

I have a white '91 N/A and my rear bumber above the exhausts were black. I'd clean it off (sometimes even had to use polishing compound to get it clean) and, within two weeks, it would be black again. Anyway, I replaced the 02 sensor and fine tuned the TPS adjustment and the problem went away.

-=Chuck=-