Small Engines How to Read a Spark Plug

Published on March 20th, 2015 | by Jacks


How to Read a Spark Plug

Being able to read a spark plug can be a valuable tuning aid. By examining the center electrode color, an experienced engine tuner can determine a great deal about the engine’s overall operating condition.

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 and wet or dry deposits on the electrode can indicate an overly-rich condition, excessive amounts of oil, or too large of a plug gap.

If the deposits are wet, it can be an indication of a breached head gasket, poor oil control from ring or valvetrain problems or an extremely rich condition – depending on the nature of the liquid present at the firing tip.

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

Spark Plug Condition Examples:

Normal Condition 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.
Dry and Wet Fouling

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.


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


The accumulation of deposits on the firing end is influenced by oil leakage, fuel quality and the engine’s operating duration.

Lead Fouling

Lead Fouling
Lead fouling usually appears as yellowish brown deposits on the insulator nose. This can not be detected by a resitsance tester at room temperature. Lead compounds combine at different temperatures. Those formed at 370-470°C (700-790°F) have the greatest influence on lead resistance.


Breakage is usually caused by thermal expansion and thermal shock due to sudden heating or cooling.

Normal Life

Normal Life
A worn spark plug not only wastes fuel but also strains the whole ignition system 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)

Abnormal Erosion

Abnormal Erosion
Abnormal electrode erosion is caused by the effects of corrosion, oxidation and reaction with lead – all resulting in abnormal gap growth.


Melting is caused by overheating. 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).

Erosion, Corrosion & Oxidation

Erosion, Corrosion and Oxidation
The material of the electrodes has oxidized, and when the oxidation is heavy it will be green on the surface. The surface of the electrodes are also fretted and rough.

Lead Erosion

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 seperating 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.

Jack’s Safety Tips: Before servicing or repairing any power equipment, disconnect the spark plug and battery cables. Remember to wear appropriate safety glasses and gloves to protect against harmful chemicals and debris. View our Disclaimer.

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About the Author

Jack's Small Engines has been providing parts for outdoor power equipment online since 1997. We also run a service center for outdoor power equipment like riding mowers, snow blowers, generators, chainsaws, and just about anything else.

4 Responses to How to Read a Spark Plug

  1. james says:

    I have a question that never seem to get brought up, what is your opinion on high test fuels, (I heard that they will make your engine run hotter) and what about lead substitute for old engines. should I run a lead substitute in older engines.

  2. Tim says:

    Hello, the “spark plug condition: examples” images are not showing. Is there anyway u can repair them? Thanks

  3. skip says:

    poulan pp4218avhd pro primer bulb 2 hoses to carb none to fuel tank what side gets the longer nip on primer bulb thank you

  4. Tom Saskiewicz says:

    I have a Craftsman tractor —circa 1989 vintage. The Tecumseh engine number is 143.389015. It’s a 12HP vertical shaft OHV engine and has about 1000 to 1200 running hours on it. I’m beginning to detect bad engine noises ,like bad bearings or a rod knock. Is 1000 hours about the life of this engine or is worth my rebuilding it ?

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