Suburban 1/2 Ton 2WD V8-454 7.4L (1987)
Spark Plug Wire: All Technical Service Bulletins
Ignition System - Diagnosis Precautions
Number:
88-138-6D
Section:
6D
Date:
Feb., 1988
Subject:
ENGINE MISS, HESITATION, OR ROUGHNESS DUE TO PIERCED SECONDARY IGNITION COMPONENTS
Model and Year:
ALL MODELS WITH GASOLINE ENGINE
TO:
ALL CHEVROLET DEALERS
During the diagnosis procedures for an engine miss, hesitation, or roughness condition, a spark plug or spark plug wire condition may be suspected.
Several types of commercial or homemade diagnostic equipment require the secondary ignition boots or wire to be pierced. This is normally done to
check for spark plug firing or to perform a cylinder balance test. Similarly, the use of pliers or other such tools to disengage a spark plug boot may pierce
or damage the boot or wire.
NOTICE:
SECONDARY IGNITION COMPONENTS SHOULD NOT BE PIERCED FOR ANY REASON.
Piercing a spark plug wire and/or distributor or direct fire module nipple may create a failure condition that will not be immediately apparent. Over time,
the hole in the pierced boot may allow a ground path to develop, creating a plug misfire condition. Heavily moisture-laden air in the vicinity of the
pierced boot may accelerate this effect.
Piercing a secondary ignition wire creates a gap in the wire's conductive core. This gap is a point of high resistance. The current flow in the wire will
increase to compensate for the higher wire resistance. Over time, the wire may fail, creating a plug misfire condition. The time required for the condition
to appear depends upon the extent of damage to the conductive core.
To help prevent future customer comments that are spark plug wire related, do not pierce or otherwise damage any secondary ignition component. Only
use diagnostic equipment containing an inductive pick-up to check for spark plug firing or to perform cylinder balance tests. When disengaging a spark
plug boot from the spark plug, twist the flanged boot 1/2 turn, then pull only on the boot to remove the wire.
