Chrysler RAM TRUCK 2500 1994 Service Manual page 377

8D - 20 IGNITION SYSTEMS
Fig. 30 Heat Shields—3.9L ¥-6 or 5.2/5.9L ¥-8
Engines
number of ways: poor fuel economy, power loss, de­
crease i n engine speed, hard starting and, i n general,
poor engine performance.
Remove the spark plugs and examine them for
burned electrodes and fouled, cracked or broken por­
celain insulators. Keep plugs arranged i n the order
in which they were removed from the engine. A n iso­
lated plug displaying an abnormal condition indicates
that a problem exists i n the corresponding cylinder.
Replace spark plugs at the intervals recommended i n
the maintenance chart i n Group 0, Lubrication and
Maintenance.
Spark plugs that have low mileage may be cleaned
and reused i f not otherwise defective. Refer to the
following Spark Plug Condition section of this group.
CONDITION
10RMAL OPEiATlie
The few deposits present on the spark plug will
probably be light tan or slightly gray i n color. This is
evident with most grades of commercial gasoline
(Fig. 31). There will not be evidence of electrode
burning. Gap growth will not average more than ap­
proximately 0.025 mm (.001 in) per 1600 k m (1000
miles) of operation. Spark plugs that have normal
wear can usually be cleaned, have the electrodes
filed, have the gap set and then be installed.
Some fuel refiners i n several areas of the United
States have introduced a manganese additive (MMT)
for unleaded fuel. During combustion, fuel with MMT
causes the entire tip of the spark plug to be coated
with a rust colored deposit. This rust color can be
misdiagnosed as being caused by coolant i n the com­
bustion chamber. Spark plug performance is not af­
fected by MMT deposits.
NORMAL DRY BLACK COLD (CARBON) FOULING
J908D-15
Fig. 31 Normal Operation and Cold (Carbon) Fouling
COLD FOULING/CARBON FOULING
Cold fouling is sometimes referred to as carbon
fouling. The deposits that cause cold fouling are ba­
sically carbon (Fig. 31). A dry, black deposit on one or
two plugs i n 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 filter or repeated short operating times
(short trips).
WET FOULING OR GAS FOULING
A spark plug coated with excessive wet fuel or oil is
wet fouled. I n older engines, worn piston rings, leak­
ing valve guide seals or excessive cylinder wear can
cause wet fouling. I n new or recently overhauled en­
gines, wet fouling may occur before break-in (normal
oil control) is achieved. This condition can usually be
resolved by cleaning and reinstalling the fouled
plugs.
OIL OR ASH ENCRUSTED
I f one or more spark plugs are oil or oil ash en
crusted (Fig. 32), evaluate engine condition for the
cause of oil entry into that particular combustion
chamber.
ELECTRODE GAP BRIDGING
Electrode gap bridging may be traced to loose de­
posits i n the combustion chamber. These deposits ac­
cumulate 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. 33).
This short circuits the electrodes. Spark plugs with
electrode gap bridging can be cleaned using standard
procedures.
SCAVENGER DEPOSITS
Fuel scavenger deposits may be either white or yel­
low (Fig. 34). They may appear to be harmful, but