My sons 03 S-10 is running rough at idle. He had a blown head gasket between cylinder 4 and 6 that has been fixed. Cleaned the Mass air flow sensor, new plugs, wires, distributor cap, rotor and fuel filter. idled fine at first then after a few starts it began to idle rough again. Runs fine when you give it gas. Engine light is on but it hasn't been reset yet since I replaced the head gasket. Anyone have any idea what else I might try?