Spark Plug Inspection: Notes

Spark Plug Usage

• Ensure that the correct spark plug is installed. An incorrect spark plug causes driveability conditions. Refer to Ignition System Specifications  for the correct spark plug.
• Ensure that the spark plug has the correct heat range. An incorrect heat range causes the following conditions:
  • Spark plug fouling-Colder plug
  • Pre-ignition causing spark plug and/or engine damage-Hotter plug

Spark Plug Inspection

Fig 1: Cross Sectional View Of Spark Plug
Courtesy of GENERAL MOTORS CORP.
• Inspect the terminal post (1) for damage.
  • Inspect for a bent or broken terminal post (1).
  • Test for a loose terminal post (1) by twisting and pulling the post. The terminal post (1) should NOT move.
Fig 2: Identifying Points For Inspecting Spark Plug For Flashover Or Carbon Tracking Soot
Courtesy of GENERAL MOTORS CORP.
• Inspect the insulator (2) for flashover or carbon tracking, soot. This is caused by the electrical charge traveling across the insulator (2) between the terminal post (1) and ground. Inspect for the following conditions:
  • Inspect the spark plug boot for damage.
  • Inspect the spark plug recess area of the cylinder head for moisture, such as oil, coolant, or water. A spark plug boot that is saturated causes arcing to ground.
Fig 3: Inspecting Spark Plug Insulator
Courtesy of GENERAL MOTORS CORP.
• Inspect the insulator (2) for cracks. All or part of the electrical charge may arc through the crack instead of the electrodes (3, 4).
Fig 4: Cutaway/Description View Of Spark Plug
Courtesy of GENERAL MOTORS CORP.
• Inspect for evidence of improper arcing.
  • Measure the gap between the center electrode (4) and the side electrode (3) terminals. Refer to Ignition System Specifications . An excessively wide electrode gap can prevent correct spark plug operation.
  • Inspect for signs of tracking that occurred near the insulator tip instead of the center electrode (4).
  • Inspect for a broken or worn side electrode (3).
  • Inspect for bridged electrodes (3, 4). Deposits on the electrodes (3, 4) reduce or eliminates the gap.
  • Inspect for worn or missing platinum pads on the electrodes (3, 4), if equipped.
  • Inspect for excessive fouling.