2005 Chevy Silverado 2500 6.0L
a while ago a P0300 code popped up and a I have a rough idle and it also skips when I’m hauling a heavy load.
it had a lean of positive 18-20
then I replaced the intake gasket and fuel injector and rail but the code and misfires still
persisted but the lean changed to a negative 8 to 11
im at a lost and not sure what I need to do

 

In most cases a tune up will fix that problem. I have also seen a clogged catalytic converter cause similar problems.