Why my 94 gmc truck won't idle and blows black smoke at start up?

sounds like the oxygen sensor is bad

if you have the vortech engine it sounds like the central fuel injector is bad. its one main injector with 8 spider like tubes that go to each cylinder not a cheap job make sure they use a delco part not aftermarket they also have to change two fuel supply hoses it happens to all vortechs 80000+ miles when you start it it paints black carbon circles on your driveway get it done or you will ruin the engine smell your dipstick it smells like gas change oil when job is done email me for more help if you need g