96 burban running very ritch?????

hi i have a 96 suburban 5.7 4x4 1500 . it seems to run very ritch. i can see fuel smooke at tail pipe and fumes are terriable. i have no service lights on and it runs ok just a littal rough. i do have a hard time getting it running when hot . i must crank it more than normal. plugs and wires and cap all newer. i have tryed fuel additives, it does run smoother after i install. but after i add fuel agian over a short time it runs rough agian. any help please. thanks. mark.

any codes?

Basic tuneup?
O2 sensor been checked?

no codes ,tune up fresh . o2 sensor.?? no codes.

faulty/lazy o2 sensor will make engine run rich

i changed the two up stream o2s about six months ago but they were causing service light and codes. is there any way to test o2 sensors . thanks.

How to test 02 sensors (http://mr2.com/TEXT/O2_Sensor.html)

thank you i will try this.