Jeep cherokee 1989 Owner's Manual page 61

The Motor Manual Guy
: Dry sooty deposits
ich mixture or weak
uses misfiring, hard
Recommendation: Check for
a clogged air cleaner, high float
level, sticky choke and worn ig-
nition points. Use a spark plug
with a longer core nose for
greater anti-fouling protection.
OIL DEPOSITS
Symptoms: Oily coating
caused by poor oil control. Oil
is leaking past worn valve
guides or piston rings into the
combustion chamber. Causes
the mechanical condition with
Symptoms: Blistered, white in-
sulator, eroded electrode and
absence of deposits. Results in
shortened plug life.
Recommendation: Check for
the correct plug heat ran.ge,
over-advanced ignition timing,
lean fuel mixture, intake mani-
fold vacuum leaks arld sticking
valves. Check the coolant level
and make sure the r'adiator is
PREIGNITION
Symptoms: Melted electrodes.
Insulators are white, but may
be dirty due to misfiring or fly-
ing debris in the combustion
the correct plug heat range,
over-advanced ignition timing,
lean fuel mixture, clogged cool-
ing system and lack of lubri-
Symptoms: Insulator has
yellowish, glazed appearance.
Indicates that combustion
chamber temperatures have
risen suddenly during hard ac-
celeration. Normal deposits
melt to form a conductive coat-
ing. Causes misfiring at high
Recommendation: Install new
plugs. Consider using a colder
plug if driving habits warrant.
GAP BRIDGING
Symptoms: Combustion de-
Recommendation: Locate the
faulty plug and remove the de-
NORMAL
Symptoms: Brown to grayish-
tan color and slight electrode
wear. Correct heat range for
engine and operating condi-
tions.
Recommendation: When new
spark plugs are installed, re-
place with plugs of the same
heat range.
ASH DEPOSITS
Symptoms: Light brown de-
posits encrusted on the side or
center electrodes or both. De-
rived from oil and/or fuel addi-
tives. Excessive amounts may
mask the spark, causing misfir-
ing and hesitation during accel-
eration.
Recommendation: If exc
sive deposits accumulate o
a short time or low mileage,
stall new valve guide seals to
prevent seepage of oil into the
combustion chambers. Also try
changing gasoline brands.
WORN
Symptoms: Rounded elec-
trodes with a small amount of
deposits on the firing end. Nor-
mal color. Causes hard starting
in damp or cold weather and
poor fuel economy.
Recommendation: Replace
with new plugs of the same
heat range.
DETONATION
Symptoms: Insulators may be
cracked or chipped. Improper
gap setting techniques can
also result in a fractured insu-
lator tip. Can lead to piston
damage.
Recommendation: Make sure
the fuel anti-knock values meet
engine requirements. Use care
when setting the gaps on new
plugs. Avoid lugging the en-
gine.
SPLASHED DEPOSITS
Symptoms: After long periods
of misfiring, deposits can
loosen when normal combus-
tion temperature is restored by
an overdue tune-up. At high
speeds, deposits flake off the
piston and are thrown against
the hot insulator, causing mis-
firing.
Recommendation: Replace
the plugs with new ones or
clean and reinstall the orig-
inals.
MECHANICAL DAMAGE
Symptoms: May be caused by
a foreign object in the combus-
tion chamber or the piston
striking an incorrect reach (too
long) plug. Causes a dead cyl-
inder and could result in piston
damage.
Recommendation: Remove
the foreign object from the en-
gine and/or install the correct
reach plug.