Engine - Miss, Hesitation, or Roughness

Number: 93-35-6D
Section: 6D
Date: OCT. 1992
Corporate Bulletin No.: 716404R
ASE No.: A1, A8

Subject:
ENGINE MISS HESITATION OR ROUGHNESS DUE TO PIERCED SECONDARY IGNITION COMPONENTS

Model and Year:
1980-93 ALL PASSENGER CARS AND TRUCKS

THIS BULLETIN CANCELS AND SUPERSEDES DEALER SERVICE BULLETIN NO. 87-121, DATED MAY 1987. THE 1989-93 MODEL YEARS HAVE BEEN ADDED. ALL COPIES OF 87-121 SHOULD BE DISCARDED.

During the diagnosis procedure for an engine miss, hesitation or roughness, a spark plug or spark plug wire condition may be suspected. Several types of commercial or homemade diagnostic equipment required 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. Secondary ignition components should not be pierced for any reason.

Piercing a spark plug wire and/or distributor boot may create a 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 condition 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 on the boot only to remove the wire.