Thread: How-To: Reading Spark Plugs

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
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.


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.


Deposits
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 usually appears as yellowish brown
deposits on the insulator nose. This can not 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.


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


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


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).


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 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.