You can actually read your spark plugs for valuable clues about how your engine is operating. If your plugs indicate that something is seriously wrong with your engine, ask a professional for an opinion.
To read a spark plug and assess engine troubles, you first have to remove it. Use a spark plug wrench with an extender. Once you get the socket over the spark plug, place your hand over the head of the wrench, grasping the head firmly and pull the handle, hitting it gently with the palm of your hand to get it going.
Once you have the plug in your hand, check out its various parts to look for tell-tale signs of trouble. The following figure can help you
identify the different parts of a spark plug.
You should also check the gap between the center electrode and the side electrode. Take your wire or taper feeler gauge and locate the proper wire and slide it into that gap. If the gauge has a lot of room to wiggle around, your old plug may have worn down its center electrode, causing a gap that’s too large. If the gauge can’t fit between the center and side electrodes, the gap is too small, which means that the spark plug isn’t burning the fuel/air mixture efficiently.
Compare the state of your spark plug with the table below to diagnose any problems.
What Your Old Spark Plugs Tell You about Your Vehicle