If you are able to read a spark plug it can be a valuable tuning aid.

An experienced engine tuner can determine a great deal about the engine’s overall operating condition by examining the insulator nose color.

In general, a light tan/gray color tells you that the spark plug is operating at optimum temperature and that the engine is in good condition. Dark coloring, such as heavy black wet or dry deposits can indicate an overly-rich condition, too cold a heat range spark plug, a possible vacuum leak, low compression, overly retarded timing or too large a plug gap.

Wet deposits may be an indication of:

  • A breached head gasket.
  • Poor oil control from rings.
  • Valve train problems.
  • An extremely rich condition.

Depending on the nature of the liquid present at the firing tip.

Look for any signs of fouling or excessive heat. This will help prevent further deterioration of performance and possible engine damage.

spark-plug condition
spark-plug condition
  • 1 Normal Condition.

  • An engine’s condition can be judged by the appearance of the spark plug’s firing end. If the firing end of a spark plug is brown or light gray, the condition can be judged to be good and the spark plug is functioning optimally.
  • 2. Dry and Wet Fouling.

  • Although there are many different cases, if the insulation resistance between the center electrode and the shell is over 10 ohms, the engine can be started normally. If the insulation resistance drops to 0 ohms, the firing end is fouled by either wet or dry carbon.
  • 3. Overheating.

  • When a spark plug overheats, deposits that have accumulated on the insulator tip melt and give the insulator tip a glazed or glossy appearance.
  • 4. Deposits.

  • Oil leakage, fuel quality and the engine’s operating duration will influence the accumulation of deposits.
  • 5. Lead Fouling.

  • Consequently, Lead fouling usually appears as yellowish brown deposits on the insulator nose. A resistance tester at room temperature will not detect this. Lead compounds combine at different temperatures. Those formed at 370-470°C (700-790°F) having the greatest influence on lead resistance.
  • 6. Breakage.

  • Hence, Thermal expansion and thermal shock due to sudden heating or cooling can cause breakage.
  • 7. Normal Life.

  • A worn spark plug not only wastes fuel but also strains the whole ignition system. This is because the expanded gap (due to erosion) requires higher voltages. Normal rates of gap growth are as follows:
    Four Stroke Engines: 0.01~0.02 mm/1,000 km (0.00063~0.000126 inches/1,000 miles)
    Two Stroke Engines: 0.02~0.04 mm/1,000 km (0.000126~0.00252 inches/1,000 miles)
  • 8. Abnormal Erosion.

  • As a result, Abnormal electrode erosion is caused by the effects of corrosion, oxidation and reaction with lead – all resulting in abnormal gap growth.
  • 9. Melting.

  • Overheating will cause melting. The electrode surface is rather lustrous and uneven. The melting point of nickel alloy is 1,200~1,300°C (2,200~2,400°F).
  • 10. Erosion, Corrosion and Oxidation.

  • The material of the electrodes has oxidized, and when the oxidation is heavy it will be green on the surface. Fretted may appear on the electrodes.
  • 11. Lead Erosion.

  • Lead erosion is caused by lead compounds in the gasoline which react chemically with the material of the electrodes (nickel alloy) as high temperatures; crystal of nickel alloy fall off because of the lead compounds permeating and separating the grain boundary of the nickel alloy. Typical lead erosion causes the surface of the ground electrode to become thinner, and the tip of the electrode looks as if it has been chipped.

Conclusion-To keep your engine operating efficiently, don’t mix plugs in varying states of wear. Either replace all the plugs with new ones or clean and reinstall all the old ones.

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