light brownish-white accumulations that form on and
around the electrode. Normally, ash deposits are non-
conductive, but large amounts may cause misfiring.
Splash Fouling
Small dark patches visible on the insulator indicate
splash fouling. Deposits breaking loose from pistons
and valves and splashing against hot plug insulators
cause splash fouling. The condition often occurs after
engine servicing that restores engine power and higher
combustion temperatures. Splash-fouled plugs can
generally be cleaned and reinstalled.
Gap Bridging
Gap bridging is usually due to conditions similar to
those described for splash fouling. The difference is that
deposits form a bridge across the electrodes and cause
a short. This condition is common in engines with poor
oil control.
Insulator Glazing
Shiny, yellow, or tan deposits are a sign of insulator glaz-
ing. Frequent hard acceleration with a resulting rise in
plug temperature can cause glazing. The high tempera-
ture melts normal plug deposits and fuses them into a
conductive coating that causes misfiring.
Overheating
Spark plug overheating is indicated by a clean, white in-
sulator tip, excessive electrode wear, or both. The insu-
lator may also be blistered. Incorrect spark plug heat
range, incorrect tightening torque, over-advanced tim-
ing, a defective cooling system, or lean air-fuel mixture
can cause overheating.
Detonation
Detonation causes increased heat and pressure in the
combustion chamber that exerts extreme loads on en-
gine parts. Fractured or broken spark plug insulators are
a sign of detonation. Over-advanced timing, lean fuel-
air mixture, low gasoline octane, and engine lugging are
contributing factors. An EGR valve that fails to open can
also cause detonation.
Preignition
Preignition
, the air-fuel charge igniting before the plug
fires, can cause severe damage to the spark plug elec-
trodes. Preignition is usually caused by combustion
chamber hot spots or deposits that hold enough heat to
prematurely ignite the air-fuel charge. Crossfiring be-
tween plug cables or a plug heat range much too hot for
the engine can also cause preignition. A loose spark
plug may also cause preignition.
Chapter
B
: Ignition System Diagnosis and Repair
40
2. Loosen each plug one or two turns with a spark
plug socket, then blow dirt away from around the
plugs with compressed air.
3. Remove the plugs keeping them in cylinder number
order for inspection.
4. When removing gasketed plugs, be sure the old
gasket comes out with the plug.
Spark Plug Inspection
Examining the firing ends of the spark plugs reveals a
good deal about general engine conditions and plug op-
eration. The insulator nose of a used plug should have
a light brown-to-grayish color, and there should be very
little electrode wear. These conditions indicate the cor-
rect plug heat range and a healthy engine. Some com-
mon spark plug conditions that indicate problems fol-
low.
Oil Fouling
Dark, wet deposits on the plug tip are caused by ex-
cessive oil entering the combustion chamber. Piston
ring, cylinder wall, and valve guide wear are likely caus-
es in a high-mileage engine. Also, a defective PCV valve
can draw oil vapor from the crankcase into the intake
and oil foul the plugs.
Carbon Fouling
Soft, black, sooty deposits on the plug end indicate car-
bon fouling. Carbon results from a plug that is operating
too cold. Check for spark plugs with an incorrect heat
range, an overly rich air-fuel mixture, weak ignition, in-
operative manifold heat control valve or thermostatic air
cleaner, retarded timing, low compression, faulty plug
wires or distributor cap. Carbon fouling may also result
from overloading due to excessive stop and- go driving.
Ash Fouling
Certain oil or fuel additives that burn during normal
combustion can create ash deposits. Ash deposits are
SPARK PLUG
SPARK PLUG
WIRE AND
BOOT
SPECIAL TOOL
TWIST AND PULL
Fig. 2-12.
Use insulated plug cable pliers to disconnect cables from
spark plugs.
Содержание ASE-A8
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