Reading a Spark Plug
Inspect the engine's spark plugs if there is a misfire, dead cylinder, or if there is a suspected mechanical failure. It is one of the first places to look when there is a problem with drivability. A spark plug can be a great indicator of what is going on inside the engine's combustion chamber. It can indicate if there is a rich or lean air-fuel ratio. It shows if oil or coolant is leaking into the cylinder.
Normal Wear: A light tan or light gray colored spark plug tip is considered normal. Today's fuel-injected engines usually turn the tip a light gray color. The center electrode should be square, not rounded; this is because spark readily jumps off a square edge.
Rich Fuel Condition: Black flakey soot around the tip and electrode indicates a rich air-fuel ratio. A dripping fuel injector results in a wet spark plug tip that smells like raw fuel.
Oil Control Condition: Black carbon or thick light brown crusty deposits indicate an oil control problem. A bad valve, valve seals, or worn piston rings result in oil burning in the combustion chamber and black carbon deposits on the spark plug tip.
Lean Fuel Condition: White carbon deposits around the electrodes and tip indicate a lean fuel condition. Overheating the spark plug results in a glazed or glossy tip.
Detonation: Detonation causes damage to the white electrode insulator. Always use fuel that contains the proper octane and make sure the EGR system is functioning correctly.