my car idles pretty rough and when i get on it to about 3500-4000 rpms it starts throwing out black smoke through the exhaust and doesn't want to accelarate any more. could this be a lack of fuel maybe it could be running lean? could this be the fule pressure regulator or if i changed the fuel filter could that help? it's a 94Z A4.

 

Black smoke would more likely indicate rich-not lean. You might need to check your pressure with a test gauge. You can also remove the vacuum line to the regulator and check it for the presence or smell of fuel. If it is leaking into the vacuum line, that can cause a rich condition. Bad O2s can cause air/fuel problems, too (as can a number of other things).

 

Black smoke probably indicating unburnt fuel, i.e, you are probably running too rich rather than too lean. Perhaps your 02 is boogered.JMHO