normal NORMAL

Brown to grayish-tan color and slight electrode wear. Correct heat range for engine and operating conditions.

When new spark plugs are installed, replace with plugs of the same heat range.

normal WORN

Rounded electrodes with a small amount of deposits on the firing end. Normal color.

Causes hard starting in damp or cold weather and poor fuel economy. The voltage required to fire the plug has approximately doubled and will continue to increase with additional miles of travel. Even higher voltage requirements, as much as 100% above normal, may occur when the engine is quickly accelerated. Poor engine performance and a loss in fuel economy are traits of a worn spark.

Carbon Fouled - Dry sooty deposits indicate and rich mixture or weak ignition. Causes misfiring, hard starting and hesitation.
Wet carbon fouling is also possible.
Ash Fouled - Light brown deposits encrusted on the side or center electrodes or both. Derived from oil and/or fuel additives. Excessive amounts may mask the spark, causing misfiring and hesitation during acceleration.
Oil Fouled - Oily coating caused by poor oil control. Oil is leaking past worn valve guides or piston rings into the combustion chamber. Causes hard starting, misfiring and hesitation
Lead Fouled - Lead fouling usually appears as yellowish brown deposits on the insulator nose. These can be detected by a resistance tester at room temperature.
Splash Fouled - Appears as "spotted" deposits on the firing tip of the insulator and often occurs after a long delayed tune-up. By-products of combustion may loosen suddenly. During hard acceleration these materials shed from the piston crown or valve heads, and are thrown against the hot insulator surface.


The accumulation of deposits is influenced by oil leakage, fuel quality and the engine's operating period.

Deposits come from:

  • Fuel - Carbon, Lead, Bromine
  • Lubricating Oil - Carbon, Calcium, Sulphur, Barium, Zinc
  • Others - Iron, Silicon, Aluminum, etc.

Blistered, white insulator, eroded electrode and absence of deposits. Results in shortened plug life.

This is often caused by over advanced ignition, timing, poor engine cooling system efficiency (scale, stoppages, low level), a very lean air/fuel mixture, or a leaking intake manifold. When these conditions prevail, even a plug of the correct heat range will overheat.

Melting may also occur. Usually, the electrode surface is rather lustrous and uneven. Nickel alloys melt at between 2200 and 2400 deg.F.


Melted electrodes. Insulators are white, but may be dirty due to misfiring or flying debris in the combustion chamber. Can lead to engine damage.

Check for the correct plug heat range, over advanced timing ignition, lean fuel mixture, insufficient engine cooling and lack of lubrication.

Usually one or a combination of several engine operating conditions are the prime causes of pre-ignition. It may originate from glowing combustion chamber deposits, hot spots in the combustion chamber due to poor control of engine heat, cross-firing (electrical induction between spark plug wires), or the plug heat range is too high for the engine or its operating conditions.


Insulator has yellowish, glazed appearance. Indicates that combustion chamber temperatures have risen suddenly during hard acceleration. Normal deposits melt to form a conductive coating which can causes misfiring at high speeds.

Install new plugs. Consider using a colder plug is driving habits warrant.


Insulators may be cracked or chipped. Improper gap setting techniques can also result in a fractured insulator tip. Can lead to piston damage. The explosion that occurs in this situation apples extreme pressures on internal engine components.

Prime causes include ignition time advanced too far, lean air/fuel mixtures, and insufficient octane rating of the gasoline. Make sure the fuel antiknock values meet engine requirements. Use care when setting the gaps on new plugs. Avoid lugging the engine.

Oxidation - The material of the electrodes has oxidized, and when oxidation is heavy it will be green on the surface. The surface of the electrodes are also fretted and rough.
Abnormal Erosion - Caused by the effects of corrosion, oxidation, reaction with lead, all resulting in abnormal gap growth.
Lead Erosion - Caused by lead compounds in the gasoline which react chemically with the material of the electrodes at high temperatures. Typical lead erosion causes the surface of the ground electrode to become thinner and the tip of the electrode will look as if it has been chipped.

May be caused by a foreign object in the combustion chamber or the piston striking an incorrect reach (too long) plug. Causes a dead cylinder and could result in piston damage.

Repair the mechanical damage. Remove the foreign object from the engine and/or install the correct reach plug. When this condition is discovered, check the other cylinders to prevent a recurrence, since it is possible for a small object to "travel" from one cylinder to another where a large degree of valve overlap exists.


Grey to Light Golden-Brown Color

This condition is ideal, the spark plug and engine air/fuel mixture are operating properly.


Black Soot Buildup

  • Air/fuel mixture is too rich, the carburetor settings are incorrect.
  • Spark plug heat range is too cold for the operating conditions.
  • Ignition system problems causing a weak or intermittent spark.
Wet Fouling:

Shiny, Wet, Black Appearance

  • Excessive use of the choke (gas fouled)
  • Prolonged low rpm operation (gas or oil fouled)
  • Fuel to oil ratio is too rich (oil fouled)
Excess Deposits:

Bumpy, Chalky Buildup

  • Poor fuel quality
  • Oil leakage into combustion chamber
  • Improper oil used for premix/injected

White, Blistered, Melted Electrode

  • Lean air/fuel mixture
  • Spark plug heat range is too hot for operating condition of the engine
  • Plug is not properly gapped and/or torqued onto head
  • Overly advanced timing

A detonation problem would show signs, such as silver specs, black specs, or melting or breakage at the firing tip.

Signs of fouling or excessive heat must be traced quickly to prevent further deterioration of performance and to prevent possible engine damage.

SOURCES: Champion Spark Plugs, NGK Spark Plugs, MechanicMatt.com, Verrill.com

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