Common spark plug conditions
NORMAL
Symptoms: Brown to grayish-tan color and slight
electrode wear. Correct heat range for engine and
operating conditions.
Recommendation:
When new spark plugs are
installed, replace with plugs of the same heat range.
WORN
Symptoms: Rounded electrodes
with a small amount of deposits on
the firing end. Normal color. Causes
hard starting in damp or cold
weather and poor. fuel economy.
Recommendation: Plugs have
been left in the engine too long.
Replace with new plugs of the
same heat range. Follow the rec-
ommended maintenance schedule.
CARBON DEPOSITS
Symptoms: Dry sooty deposits
indicate a rich mixture or weak igni-
tion. Causes misfiring, hard starting
and hesitation.
Recommendation: Make sure the
plug has the correct heat range.
Check for a clog
9
ed air filter or
problem in the uel system or
engine management system. Also
check for ignition system problems.
ASH DEPOSITS
Symptoms: Light brown deposits
encrusted on the side or cenfer
electrodes or both. Derived from oil
and/or fuel additives. Excessive
amounts may mask the spark,
causing misfiring and hesitation
during acceleration.
Recommendation: If excessive
deposits accumulate over a short
time or low mileage, install new
valve guide seals to prevent seep-
age of oil into the combustion
chambers. Also try changing gaso-
line brands.
OIL DEPOSITS
Sympfoms: Oily coating caused
by poor oil control. Oil is leaking
past worn valve guides or piston
rings into the combustion chamber.
Causes hard starting, misfiring and
hesitation.
Recommendation: Correct the
mechanical condition with neces-
sary repairs and install new plugs.
GAP BRIDGING
Symptoms: Combustion deposits
lodge between the electrodes.
Heavy deposits accumulate and
bridge the electrode gap. The plug
ceases to fire, resulting in a dead
cylinder.
Recommendation: Locate the
faulty plug and remove the deposits
from between the electrodes.
I
TOO HOT
Symptoms: Blistered, white insu- F____
lator, eroded electrode and
absence of deposits. Results in
shortened plug life.
Recommendation: Check for the
correct plug heat range, over-
advanced ignition timing, lean fuel
mixture, intake manifold vacuum
leaks, sticking valves and insuffi-
cient engine cooling.
PREIGNITION
Symptoms: Melted electrodes.
Insulators are white, but may be
dirty due to misfiring or flying
debris in the combustion chamber.
Can lead to engine damage.
Recommendation: Check for the
correct plug heat range, over-
advanced ignition timing, lean fuel
mixture, insufficient engine cooling
and lack of lubrication.
HIGH SPEED GLAZING
Symptoms: Insulator has yellow-
ish, glazed appearance. Indicates
that combustion chamber temper-
atures have risen suddenly during
hard acceleration. Normal deposits
melt to form a conductive coating.
Causes misfiring at high speeds.
Recommendation: Install new
plugs. Consider using a colder plug
if driving habits warrant.
DETONATION
Symptoms: Insulators may be
cracked or chipped. Improper gap
setting techniques can also result
in a fractured insulator tip. Can lead
to piston damage.
Recommendation: Make sure the
fuel anti-knock values meet engine
requirements. Use care when set-
ting the gaps on new plugs. Avoid
lugging the engine.
MECHANICAL DAMAGE
Symptoms: May be caused by a
foreign object in the combustion
chamber or the piston striking an
incorrect reach (too long) plug. )
Causes a dead cylinder and could
result in piston damage.
Recommendation: Repair the
mechanical damage* Remove the
foreign object from the engine
and/or install the correct reach
plug.
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