Chevy silver ado 2004 has a code po300.

Question

I changed plugs, wires, map air flow, complete head gasket kit, 4 o2 sensors, new cat.,coil packs and I seem to get the check engine light when I let it idle.

Answer 1

P0300 Random misfire,

Random misfire just means more that 2 cylinders, You may still have a fuel pressure regulator that is vacuum controled pull the hose off it and see if gas comes out if it does there is the cause of the random misfire, This year may still be equipt with a EGR, at idle feel it, if its hot to touch its open and will cause a ransom misfire, Another test you can do is test for back pressure, Some year do not have EGR and a plugged exhaust will send inert gas back to the intake,and cause P0300, So do a power brake test, it it stumble verify for a plugged exhaust seperate where the exhaust and the cat meet and retest.