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Spark Plug
Firing End Analysis |
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| Standard Spark Plug Conditions |
| An examination of used spark plugs can reflect
the accuracy of heat range application. Close scrutiny of the firing
end can also suggest a variety of possible engine conditions and
point the way to achieving improved spark plug and engine
performance. The quantity, color or overall appearance, as well as
the chemical make-up of the deposits, are all important in making
the correct diagnosis.
Remember that the electrical properties of
deposits at operating temperatures in the engine may be entirely
different than the properties at normal ambient temperatures.
Deposits normally occur in distinct bands on the
insulator. These reflect the different temperatures which occur
between the hot insulator tip and the relatively cool insulator
base.
Generally speaking, tip deposits near the
electrodes are the ones that control spark plug operation. If they
become conductive, their "shunt resistance" is said to drop...and
the plug short circuits.
Illustrations in this section include common spark
plug appearances as well as those encountered only rarely. |
| Firing End Analysis |
| 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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