HELP!!!!! my stock GTS with the 4af is only fireing on 3 cylinders, being a diesel mechanic i have almost no idea what to do with this problem on a gasoline engine. only thing i know is it may damage the cat, i looked at the sparkplugs and theyre all fine except in the cylinder that isnt fireing, its black and coverd in carbon. could it be loss of compresion or a cracked head gasket over that cylinder? is it ok to drive? anny answers will be apreciated.