Spark Plugs — How They Work

The spark plug is the component that actually ignites the air-fuel mixture inside the combustion chamber. It threads into the cylinder head with its electrode tip extending into the chamber. When the PCM commands the ignition coil to fire, a high-voltage surge — 20,000 to 100,000 volts — jumps across the small gap between the center electrode and the ground electrode at the tip of the plug. That spark ignites the compressed mixture and the power stroke begins.

A spark plug that has been in the engine tells a story about what is happening in that cylinder. Light tan or gray deposits on the tip — normal combustion, everything is working correctly. Black sooty deposits — rich mixture. That cylinder is getting too much fuel or not enough air. Wet oily deposits — oil is getting into the combustion chamber. Worn rings, worn valve seals, or a failed PCV system. White blistered electrodes — the plug is running too hot. Lean condition, incorrect plug heat range, or ignition timing too advanced. Learning to read spark plugs is learning to read the engine.

Replace spark plugs at the manufacturer's recommended interval. Copper plugs typically last 30,000 miles. Platinum plugs last 60,000 to 100,000 miles. Iridium plugs can last 100,000 miles or more. Always install the exact plug type and heat range the manufacturer specifies. The wrong heat range causes either fouling from running too cold or pre-ignition damage from running too hot. When installing plugs, apply anti-seize to the threads — especially in aluminum heads — and torque to the manufacturer's specification. Overtightening strips the threads in the head. Undertightening allows combustion gases to leak past the plug seat.