Reading Your Spark Plugs

An engine's condition can be judged by the appearance of the spark plug's firing end. If the firing end is brown or light gray, the condition is good and the spark plug is functioning optimally.

If insulation resistance between the center electrode and shell is over 10 ohms, the engine can start normally. If resistance drops to 0 ohms, the firing end is fouled by wet or dry carbon.

Overheated plugs have deposits that melt and give the insulator tip a glazed or glossy appearance.

Deposit accumulation is influenced by oil leakage, fuel quality, and engine operating time.

Lead fouling shows as yellowish brown deposits on the insulator nose. Formed at 370–470°C (700–790°F) and not detected by resistance tester at room temperature.

Usually caused by thermal expansion and shock from sudden heating or cooling.

Worn spark plugs waste fuel and strain the ignition system due to expanded gap from erosion.

• 4-Stroke Engines: 0.01–0.02 mm/1,000 km (0.00063–0.000126 in/1,000 mi)
• 2-Stroke Engines: 0.02–0.04 mm/1,000 km (0.000126–0.00252 in/1,000 mi)

Caused by corrosion, oxidation, and reaction with lead—results in abnormal gap growth.

Caused by overheating; electrode surface is lustrous and uneven. Nickel alloy melts at 1,200–1,300°C (2,200–2,400°F).

Electrodes oxidize and turn green when severe. Surface becomes fretted and rough.

Caused by lead in gasoline reacting with electrode nickel alloy at high temps. Nickel crystals fall off, ground electrode thins and tip appears chipped.