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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. |
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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. |
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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. |
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Deposits
The accumulation of deposits on the firing end is influenced by oil leakage,
fuel quality and the engine's operating duration. |
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Lead Fouling
Lead fouling usually appears as yellowish brown deposits on the insulator
nose. This cannot be detected by a resistance tester at room temperature.
Lead compounds combine at different temperatures. Those formed at
370-470°C (700-790°F) having the greatest influence on lead
resistance. |
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Breakage
Breakage is usually caused by thermal expansion and thermal shock due
to sudden heating or cooling. |
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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) |
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Abnormal Erosion
Abnormal electrode erosion is caused by the effects of corrosion, oxidation
and reaction with lead - all resulting in abnormal gap growth. |
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Melting
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°C
(2,200~2,400°F). |
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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. |
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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. |
