Hi my 2006 cobalt has an unknown tune in it meaning i dont know what tune is in. It was misfiring badly so i changed the plugs and one of them was cracked. So i replaced all 4 and took it out for a drive and it ran really well, i took off like a jet down the road but soon after it misfired again when i stepped on it and fouled the plugs again. Anyone have any answers would be very appreciated thank you