Spark Plug – How Do I Read My Spark Plug Condition – What They Tell You

Spark Plug - How Do I Read My Spark Plug Condition - What They Tell You
Spark Plug - How Do I Read My Spark Plug Condition - What They Tell You

Learning how 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 color of a spark plug.

So, if your vehicle has a little less spunk lately, it might be time to change your spark plugs.

A quick inspection can not only tell you if it’s time for spark plug replacement; but can speak volumes about your engines condition as well.

Reading The Spark Plugs Condition
Reading The Spark Plugs Condition

Is your engine running too lean? Too rich? Is there an issue with, oil control or ignition timing? Or is everything just fine with your engine?

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.

What A Spark Plug Can You About Your Engines Condition

Condition Clues Probable Causes Remedies
Normal plug Brown or grayish- tan deposit on side electrode, Everything’s fine. Hence, Just clean and re-gap the plug.
Carbon-fouled plug Black, dry, fluffy soot on insulator tip and electrodes. Overly rich fuel/air mixture, dirty air filter, too much
driving at low speeds, or idling for a long time.
Switch to “hotter” plug. (The higher the plug number, the hotter the plug.)
Oil-fouled plug Wet, black, oily deposits on insulator tip and electrodes. Oil may be leaking into cylinders past worn pistons or poorly
adjusted or worn valves.
Clean and re-gap the plug, or replace it, but find out where the leak is coming from.
Burned plug Blisters on insulator tip, melted electrodes, burned
Engine overheating, gap is too wide, wrong or loose plugs,
overly lean fuel/air mixture, or incorrect timing
Hence, Replace the plug.
Worn plug Severely eroded or worn electrodes Plug has been in there too long Hence, Replace the plug.

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 type of the liquid, present at the firing tip.

Also, look for any signs of fouling or excessive heat. Because, this will help prevent, further deterioration of performance and possible engine damage.

Spark Plug Condition
Spark Plug Condition

1. Normal Condition.

  • So, an engine’s condition can be judged by, the appearance of the spark plug firing end. Consequently, if the firing is brown or light gray, the condition can be judged as good. Consequently, 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. But, 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. As a result, giving the insulator tip, a glazed or glossy appearance.

4. Deposits.

  • Consequently, 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. So, a resistance tester at room temperature, will not detect this. Because, lead compounds combine at different temperatures. Finally, 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.

8. Abnormal Erosion.

  • So, abnormal electrode erosion, is caused by the effects of corrosion, oxidation and reaction with lead. Finally, resulting in abnormal gap growth.

9. Melting.

  • Above all, overheating will cause spark plug melting. As a result, the electrode surface is rather lustrous and uneven.

10. Erosion, Corrosion and Oxidation.

  • The material of the electrode has oxidized, and when the oxidation is heavy; it will be green on the surface. As a result, the surface of the electrodes are also fretted and rough.

11. Lead Erosion.

  • So, the tip of the ground electrode looks chipped and its surface may be thinned. Another condition unique to leaded gasoline; this condition is caused by lead compounds that; react chemically with the electrodes at high temperatures. Furthermore, this makes the electrode material (nickel alloy) weak and brittle. Consequently, this is caused by, too much lead in the fuel.


So, 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.

Thank You !