Reading a Spark Plug

Spark Plug Readings

Inspect the engine’s spark plugs if there’s a misfire, dead cylinder or if there is a suspected mechanical failure. It’s one of the first places to look when there’s a problem with drivability. This is because a spark plug can be a great indicator of what’s going on inside the engine’s combustion chamber. It can indicate if there’s a rich or lean air fuel ratio. It will show if there’s oil or coolant leaking into the cylinder.

Normal: A light tan or light gray color is normal. Today’s engines normally turn the plugs a light gray or light tan color. These are normal color readings. A worn spark plug center electrode is rounded; it should be square because spark readily jumps off a square edge.

Rich Fuel Condition: Black flakey soot around the tip and electrode indicates a rich fuel condition. A wet spark plug tip can be caused by oil, excessive fuel, or coolant. A dripping fuel injector will result 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.

Lean Fuel Condition: White carbon deposits around the electrodes and tip indicate a lean fuel condition. Overheating leaves the tip glazed and glossy.

Detonation: Damage to the white electrode insulator indicates detonation. Always use fuel that contains the proper octane and make sure the EGR system is functioning properly.