Ive got an idea for you, start with the first plug, take off the wire, unscrew the plug, put the wire back on the plug, and start the car. It WILL idle Rough cause its runnin on 3 cylinders but lay the plug and wire on the valve cover so that the plug sparks. Check for a consistant spark. If it has any break or non-constant rhythm, then you know it needs to be replaced. BE CAREFUL NOT TO TOUCH IT WHILE ITS ON!!! If you do it wil shock the $h1t out of you. Do that one at a time to see if you have a firing problem!