NORMAL |
OIL FOULED |
CARBON FOULED |
A normal condition is shown
below. This plug has been running at the correct temperature in a
"healthy" engine. Operating in such a desirable environment results in
deposits that will be light tan or gray in color with most grades of
commercial gasoline.
If LP Gas or natural gas has been used, the color will be almost brown. |
A spark plug shorted by excessive
oil entering the combustion chamber is shown below. This is often
caused by piston rings or cylinder walls that are badly worn. Oil may
also be pulled into the chamber because of excessive clearance in the
valve stem guides, or badly worn valve stem seals. If the PCV valve is
plugged or inoperative, it can cause a buildup of crankcase pressure.
This condition can force oil and oil vapors past the rings and valve
guides into the combustion chamber. |
Basically, soft, sooty carbon
deposits, as shown below, have a dry, black appearance. If only one or
two plugs in a set are fouled, it is a good practice to check for
sticking valves, a cracked distributor cap, or bad secondary ignition
wires. Fouling of the entire set might result from an incorrect heat
range spark plug or an over-rich air/fuel mixture caused by a clogged
air cleaner filter element, a sticking heat riser valve, or a faulty
choke. Fuel injectors that malfunction can also lead to this condition.
Other causes include weak ignition system
voltage or an inoperative pre-heating system (carburetor intake air) or
poor cylinder compression. |
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WORN |
INSULATOR GLAZING |
MECHANICAL DAMAGE |
This spark plug shown below has
served its useful life and should be replaced. Voltage required to fire
the plug has approximately doubled and will continue to increase the
longer the engine operates.
Even higher voltage requirements (as much as
100%) above normal may occur when the engine is accelerated quickly.
Poor engine performance and a loss of fuel economy are traits of worn
spark plugs. |
This condition may cause
misfiring at high engine RPM. Shiny deposits usually suggest that
temperatures have suddenly increased during hard acceleration. As a
result, normal metallic deposits do not have a chance to slough off the
plug and they melt and form a conductive coating which causes the
misfire. Yellow or tan deposits, as shown below, usually indicate the
use of leaded fuel. |
Mechanical damage to the firing
end, as shown below, is caused by some foreign object in the combustion
chamber. Since small objects can travel from one cylinder to another
(because of valve overlap), the other cylinders should always be checked
to prevent reoccurance of damage.
When working on an engine, it is advisable to
keep the carburetor throat (or throttle body openings on central type
fuel injected engines) covered. That precaution also applies to spark
plug holes. |
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OVERHEATED |
SPLASHED FOULED |
ASH FOULED |
An example of overheating is
shown below. Note the dead white or gray insulator nose which appears
"blistered." Electrode gap wear rate will be considerably in excess of
that normally expected. This is often caused by overadvanced ignition
timing, poor engine cooling system efficiency (scale, stoppages, low
level), a very lean A/F mixture, a leaking intake manifold, or the use
of a spark plug too hot for the application. |
Splashed fouling, as shown below,
may sometimes occur after a long-delayed tune-up. Here, deposits
accumulated after long periods of misfiring or low power operation may
be suddenly loosened when normal combustion temperatures are restored
after a set of new plugs has been installed. During a high-speed run,
these materials shedding off the piston are thrown against the hot
insulator surfaces. |
As shown below, a buildup of
combustion deposits stems primarily from the burning of oil and/or fuel
addatives during normal combustion.
These are normally nonconductive. However,
when heavier deposits are allowed to accumulate over long mileage
periods, they can "mask" the spark, resulting in a plug misfire
condition. |
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GAP BRIDGING |
PREIGNITION |
DETONATION |
An example of gap bridging is
shown below. It rarely occurs in automotive engines. Gap bridging is
caused by conditions similar to those that produce splash fouling.
Combustion deposits thrown loose may lodge between the electrodes,
causing a dead short and misfire. Fluffy materials that accumulate on
the side electrode may melt to bridge the gap when the engine is
suddenly put under a heavy load. |
As shown below, this condition
produces melting of the center electrode, and somewhat later, the ground
electrode and insulator. Usually one or a combination of several engine
operating conditions are the prime causes of preignition. 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. |
As shown below, this form of
abnormal combustion has fractured the insulator nose of the spark plug.
Explosions that occur when the operating condition exists apply extreme
pressure on internal engine components. Major causes include a faulty
EGR valve, lean air/fuel mixtures, ignition timing advanced too far, and
insufficient octane rating of the gasoline. |
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