Seville STS V8-4.6L VIN 9 (1998)
Deposit fouling of the spark plug occurs when the normal red-brown, yellow or white deposits of combustion by-products become sufficient to cause
misfiring. In some cases, these deposits may melt and form a shiny glaze on the insulator around the center electrode. If the fouling is found in only one
or two cylinders, valve stem clearances or intake valve seals may be allowing excess lubricating oil to enter the cylinder, particularly if the deposits are
heavier on the side of the spark plug that was facing the intake valve.
Excessive gap means that the airspace between the center and side electrodes at the bottom of the spark plug is too wide for consistent spark plug firing.
This may be due to improper gap adjustment or to excessive wear of the electrodes during use. Check of the gap size and compare the gap measurement
to that specified for the vehicle. Refer to Spark Plug Usage. Excessive gap wear can be an indication of continuous operation at high speeds or with
high engine loads, causing the spark plug to run too hot. Too small of a gap indicates the plug was damaged at the time of installation. Another possible
cause is an excessively lean fuel mixture.
Low or high spark plug installation torque or improper seating of the spark plug can result in the spark plug running too hot and cause excessive gap
wear. The spark plug and cylinder head seats must be in good contact for proper heat transfer and spark plug cooling. Dirty or damaged threads in the
head or on the spark plug can keep the spark plug from seating even though the proper torque is applied. Once the spark plugs are properly seated,
tighten the spark plug to the proper torque. Refer to Spark Plug Usage. Low torque may result in poor contact of seats due to a loose spark plug.
Overtightening may cause the spark plug shell to be stretched and also result in poor contact between seats.
In extreme cases, exhaust blow-by and damage beyond simple gap wear may occur.
