Jeep 1997 Grand Cherokee Service Manual page 352

8D - 10 IGNITION SYSTEM
DIAGNOSIS AND TESTING (Continued)
COLD FOULING/CARBON FOULING
Cold fouling is sometimes referred to as carbon
fouling. The deposits that cause cold fouling are basi-
cally carbon (Fig. 15). A dry, black deposit on one or
two plugs in a set may be caused by sticking valves
or defective spark plug cables. Cold (carbon) fouling
of the entire set of spark plugs may be caused by a
clogged air cleaner element or repeated short operat-
ing times (short trips).
WET FOULING OR GAS FOULING
A spark plug coated with excessive wet fuel or oil
is wet fouled. In older engines, worn piston rings,
leaking valve guide seals or excessive cylinder wear
can cause wet fouling. In new or recently overhauled
engines, wet fouling may occur before break-in (nor-
mal oil control) is achieved. This condition can usu-
ally be resolved by cleaning and reinstalling the
fouled plugs.
OIL OR ASH ENCRUSTED
If one or more spark plugs are oil or oil ash
encrusted (Fig. 16), evaluate engine condition for the
cause of oil entry into that particular combustion
chamber.
Fig. 16 Oil or Ash Encrusted
ELECTRODE GAP BRIDGING
Electrode gap bridging may be traced to loose
deposits in the combustion chamber. These deposits
accumulate on the spark plugs during continuous
stop-and-go driving. When the engine is suddenly
subjected to a high torque load, deposits partially liq-
uefy and bridge the gap between electrodes (Fig. 17).
This short circuits the electrodes. Spark plugs with
GROUND CENTER ELEC- TRODE ELECTRODE
electrode gap bridging can be cleaned using standard
procedures.
Fig. 17 Electrode Gap Bridging
SCAVENGER DEPOSITS
Fuel scavenger deposits may be either white or yel-
low (Fig. 18). They may appear to be harmful, but
this is a normal condition caused by chemical addi-
tives in certain fuels. These additives are designed to
change the chemical nature of deposits and decrease
spark plug misfire tendencies. Notice that accumula-
tion on the ground electrode and shell area may be
heavy, but the deposits are easily removed. Spark
plugs with scavenger deposits can be considered nor-
mal in condition and can be cleaned using standard
procedures.
MINIMUM
250 Ohms Per Inch
3000 Ohms Per Foot
Fig. 18 Scavenger Deposits
ERED WITH WHITE OR YEL- GROUND ELECTRODE COV- LOW DEPOSITS CENTER ELECTRODE DEPOSITS
ZG
MAXIMUM
1000 Ohms Per Inch
12,000 Ohms Per Foot