guys same problem misfire codes and problem here. Car misfires heavily at idle lots of black smoke poor MPG. We replaced rotor, cap, wires, a lot of plugs, coil, coil module, inatake valve seals, crank position sensor, and still the same problem. Intake gasket has been replaced 5 times. We tried the GM OEM twice one the autozone one the advance auto parts one, even a home made one out of gasket material. The car will run fine upon the change of the intake gasket (it even clears codes automatically) but within 24 to 72 hrs it will end up in the same condition.