4.8 liter vortec fuel problem

I have a 2001 silverado z71 with 4.8 liter vortec engine. It runs good but the tailpipe has a black sooty substance. I replaced the stock air filter with an k&n and punched out the mass air flow screen which gave more power. I also replaced one of the O2 sensors which stopped throwing fuel out the exhaust for a coupe of weeks. Any one else experience this problem? Could it be my muffler or catlytic converter maybe? Thanks! M

I'm not a expert, and look as if 20 people have looked at this without reply, so I'll make some suggestions, hahahaha....now all the armchair QB's will come out and you'll have more opinons than ever......

I would say it's running rich! are you getting better than 14 mpg?
Try dropping a K&N filter in if you have a 4.8 or 5.3 it would be a 33-2129, if you have a 6.0 33-2135....clean the airbox out, check screen on the MAF too....good luck....JT

I'm not a expert, and look as if 20 people have looked at this without reply, so I'll make some suggestions, hahahaha....now all the armchair QB's will come out and you'll have more opinons than ever......

I would say it's running rich! are you getting better than 14 mpg?
Try dropping a K&N filter in if you have a 4.8 or 5.3 it would be a 33-2129, if you have a 6.0 33-2135....clean the airbox out, check screen on the MAF too....good luck....JT
Thanks for the reply Lonewolf. I have replaced the stock air filter with an k&n and took out the mass air sensor screen. It made it run better but still gettin 13 mpg. I also replaced one of the O2 sensors on the exhaust side. That helped for a while but the unburned fuel starting showing on the exhaust pipe within two weeks.

No Problem, welcome, what size engine is it?, I'll go with the Filter, the FM 70, I'm avoiding removing parts, these new vehicles with all the computers and such, seems like the more you tinker, deep you get, try one of those computer tools everyone talks about how you can tune, I know even on the Harley with Fuel Injection, when I put the screamin eagle pipe and K&N I had to have them download a computer chip from internet, load it into my computer on bike before would run right, so never know!

The MAF screens on older Port Fuel Inj. motors cannot be removed or they will not run right. You know the kind of screen that mounts at the throttle body right before the MAF. GM had a TSB out on this. The screen directs the air around the MAF. Don't know if yours is like that, but the wrong airflow reading or trash on the small sensor wire (K&N Oil) will make it run rich or lean.