Dodge ram truck 3500 1996 Service Manual page 398

8D - 16 IGNITION SYSTEM _ — _ - _
DIAGNOSIS AND TESTING (Continued)
GROUND
J908D-11
Fig. 34 Electrode Gap Bridging
SCAVENGER DEPOSITS
Fuel scavenger deposits may be either white or yel­
low (Fig. 35). 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.
GROUND
ELECTRODE
COVERED
J908D-12
Fig. 35 Scavenger Deposits
CHIPPED ELECTRODE INSULATOR
A chipped electrode insulator usually results from
bending the center electrode while adjusting the
spark plug electrode gap. Under certain conditions,
severe detonation can also separate the insulator
from the center electrode (Fig. 36). Spark plugs with
this condition must be replaced.
' rs^ro)
GROUND
ELECTRODE
J908D-13
Fig. 36 Chipped Electrode Insulator
PREMONITION DAMAGE
Preignition damage is usually caused by excessive
combustion chamber temperature. The center elec­
trode dissolves first and the ground electrode dis­
solves somewhat latter (Fig. 37). Insulators appear
relatively deposit free. Determine i f the spark plug
has the correct heat range rating for the engine.
Determine i f ignition timing is over advanced or i f
other operating conditions are causing engine over­
heating. (The heat range rating refers to the operat­
ing temperature of a particular type spark plug.
Spark plugs are designed to operate within specific
temperature ranges. This depends upon the thick­
ness and length of the center electrodes porcelain
insulator.)
GROUND
ELECTRODE
CENTER
ELECTRODE
DISSOLVED
J908D-14
Fig. 37 Preignition Damage