GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC P42 Commercial Chassis 🢒 Engine 🢒 Engine Electrical 🢒 Spark Plug Visual Diagnosis
Table 1: Brown to Grayish-Tan Deposits and Slight Electrode Wear
Table 2: Dry, Fluffy Black Carbon Deposits
Table 3: Wet, Oily Deposits with Very Little Electrode Wear
Table 4: Colored Coatings on Insulator/Engine Misfires
Table 5: Heavy Colored Coating Deposits
Table 6: Shiny Yellow Glaze Coating on Insulator
Table 7: Burned or Blistered Insulator Tips and Badly Eroded Electrodes
Table 8: Broken or Cracked Insulator Tips

Brown to Grayish-Tan Deposits and Slight Electrode Wear

Problem

Action

The spark plug has normal wear.

Clean, regap, and reinstall the spark plug. Refer to Spark Plug Usage .

Dry, Fluffy Black Carbon Deposits

Problem

Action

The spark plug has poor ignition output.

Check the distributor to coil connections. Refer to Enhanced Ignition System Diagnosis in Engine Controls.

Wet, Oily Deposits with Very Little Electrode Wear

Problem

Action

Break-in of a new or recently overhauled engine.

Degrease, clean, and reinstall the spark plugs. Refer to Spark Plug Replacement .

The clearances of the valve stem guides are excessive.

Refer to Cylinder Head Clean and Inspect in Engine Mechanical.

The intake valve seals are worn.

Replace the valve seals. Refer to Valve Stem Oil Seal and Valve Spring Replacement in Engine Mechanical.

Colored Coatings on Insulator/Engine Misfires

Problem

Action

DEFINITION: The spark plug has red, brown, yellow, and white colored coatings on the insulator. The engine misfires intermittently under severe operating conditions.

The coatings are the by-products of combustion.
  1. Clean, regap, and reinstall the spark plug. Refer to Spark Plug Usage .
  2. If the plug is heavily coated, replace the spark plug. Refer to Spark Plug Replacement .

Heavy Colored Coating Deposits

Problem

Action

DEFINITION: Heavy colored coating deposits on the portion of the spark plug that projects into the chamber, and on the side facing the intake valve.

The valve seals are leaking (if this condition is found in only one or two cylinders).
  1. Check the valve seals. Replace the seals as necessary. Refer to Valve Stem Oil Seal and Valve Spring Replacement in Engine Mechanical.
  2. Clean, regap, and reinstall the spark plugs. Refer to Spark Plug Usage .

Shiny Yellow Glaze Coating on Insulator

Problem

Action

The coating is caused from the melted by-products of combustion.
  1. Avoid sudden acceleration or wide open throttle after long periods of low speed driving.
  2. Replace the spark plugs. Refer to Spark Plug Replacement .

Burned or Blistered Insulator Tips and Badly Eroded Electrodes

Problem

Action

The spark plugs are overheating.
    • Check the cooling system. Refer to Engine Overheating in Engine Cooling.
    • Check for a lean air/fuel mixture. Refer to Powertrain On Board Diagnostic (OBD) System Check in Engine Controls.
    • Check the heat range of the spark plugs. The plugs may be too hot. Refer to Spark Plug Usage .
    • Check ignition timing. The timing may be over-advanced. Refer to Ignition System Description .
    • Check the torque value of the spark plugs in order to ensure good plug to engine seat contact. Refer to Fastener Tightening Specifications .

Broken or Cracked Insulator Tips

Problem

Action

The plugs show heat shock from a sudden rise in tip temperature under severe operating conditions.

Replace the spark plugs. Refer to Spark Plug Replacement .

The spark plugs are improperly gapped.

Gap the plug to specification. Refer to Spark Plug Usage .