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2003 GMC 5.3 Random misfire

My son has a GMC pickup-91,00 miles. He has codes for random, multiple misfires. It idles rough, but will accelerate fairly well-dealer says it the CAT and O2 sensors-He has replaced all O2 sensors,MAF,MAP,plugs,plug wires,and put fresh gas with injector cleaner added-no difference. He gets black smoke out the tailpipe when accelerating-Any ideas? Thanks

The black smoke & misfiring are likely related - guess that the engine is running very rich. Check out the fuel system - check fuel pressure; fuel pressure regulator; fuel injectors. Also keep an eye on the condition of the oil - if it is running this rich he’s probably getting a lot of fuel in the oil.