How to Read a Spark Plug

Briggs and Stratton Spark Plug

How to Read a Spark Plug

This DIY Guide will teach you how to read a spark plug. If you have questions or comments please feel free to contact us using our contact form.

Being able to read a spark plug can be a valuable engine tuning aid. By examining the insulator firing nose color, an experienced engine mechanic 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, 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.

If the deposits are wet, it can be an indication of a breached head gasket, poor oil control from ring or valve train 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.

Normal Spark Plug
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.


Spark Plug with Dry and Wet FoulingDry 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.




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



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



Spark Plug with 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 degrees C (700-790 degrees F) having the greatest influence on lead resistance.


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



Spark Plug at 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)


Spark Plug with 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.



Melted Spark Plug
Melting is caused by overheating. Mostly, the electrode surface is rather lustrous and uneven. The melting point of nickel alloy is 1,200~1,300 degrees C (2,200~2,400 degrees F).




Spark Plug with Erosion, Corrosion and 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.




Spark Plug with 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.



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We carry a large selection of Spark Plugs. If you need help finding your Small Engine Parts, please complete the Lawn Mower Parts Request Form and we will be happy to assist you.

– Jack

Note: Images Provided by NGK Spark Plugs

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How to read a spark plug
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How to read a spark plug
Your spark plug can tell you exactly how your your engine is running. Being able to read a spark plug can be a valuable engine tuning aid. By examining the insulator firing nose color you can determine a great deal about the engine's overall operating condition.
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About Jack Hayes 75 Articles
Jack Hayes is the Head of Internet Marketing at LawnMowerPros and the editor of this blog. You can often find him creating featured articles, DIY guides, custom graphics and much more.