i noticed black soots on my rear bumper now...does that mean i am running rich?

how to fix this? oxygen sensor?

 

is your car running ok?
are you passing smog checks?
burning any oil?

if everything is ok then the black soot (carbon deposit) is generally a by product of combustion.

you can try working some seafoam in the intake. maybe a fresh tune up and some fuel injector cleaner.

other than that wash your car more frequently?