I have a 93 Jimmy with a 4.3 engine. The problem I am having is that the truck don't want to idle smooth.There is black smoke coming out of the tail pipe if you speed up the engine. I have changed plugs, One side was black from soot.I have checked the fast idle valve,The EGR valve,Changed the Dist. cap, and rotor.Checked air filter,repaired one vacuum hose.Just put wires on it about 2 months ago.What can it be? I need help.Question what does the oxgen sencor. do? And could it have anything to do with my problem.