GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC C-Series Medium Duty Truck B7 🢒 Engine 🢒 Engine Controls - 7.4L 🢒 Spark Plug Visual Diagnosis

Notice: These spark plugs have a ceramic insulator that is 3.175 mm (1/8 in) longer than earlier model spark plugs. Use a spark plug socket that is deep enough for these longer plugs (J 39358). Failure to do so could cause cracking of the insulator and arcing inside the plug resulting in an engine misfire.

Resistor type spark plugs with tapered seats are used. No gasket is needed on these plugs. These spark plugs are a High Efficiency (HE) spark plug that has a fine wire electrode, nickel plated shell and a platinum pad across from the electrode.


Object Number: 34390 Size: SH
    • Normal service is a mixture of idling, low speed, and high speed operation.
       - Occasional high speed driving is essential to good spark plug performance.
       - Increased combustion heat burns away excess carbon deposits, which accumulate from frequent idling or stop-and-go driving.
    • The heat resistant, insulating boot covers the spark plug terminal.
       - The boot extends over a portion of the spark plug insulator.
       - These boots prevent engine misfiring.
    • Corona discharge is a steady blue light appearing (in darkness) around the insulator, just above the shell crimp.
       - Do not mistake this for a shorted insulator.
       - This is visible evidence of a high-tension field.
       - This has no effect on ignition system performance.
       - This discharge repels dust particles.
       - This leaves a clear ring on the insulator just above the shell.
       - This is not evidence that combustion gases are escaping.

Problem

Action

Brown to Grayish-Tan Deposits and Slight Electrode Wear

The spark plug has normal wear.

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

Dry, Fluffy Black Carbon Deposits

The spark plug has poor ignition output.

Inspect the spark plug wires and coil connections.

Wet, Oily Deposits with Very Little Electrode Wear

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 Cleaning and Inspection in Engine Mechanical - 7.4L.

The intake valve seals are worn.

Replace the valve seals. Refer to Valve Stem Oil Seal and Valve Spring Replacement in Engine Mechanical - 7.4L.

Colored Coatings on Insulator/Engine Misfires

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 Replacement .
  2. If the plug is heavily coated, replace the spark plug. Refer to Spark Plug Replacement .

Heavy Colored Coating Deposits

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. Inspect the valve seals. Replace the seals as necessary. Refer to Valve Stem Oil Seal and Valve Spring Replacement in Engine Mechanical - 7.4L.
  2. Clean, regap, and reinstall the spark plugs. Refer to Spark Plug Replacement

Shiny Yellow Glaze Coating on Insulator

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

The spark plugs are overheating.
    • Inspect the cooling system.
    • Test for a lean air/fuel mixture. Refer to Engine Controls.
    • Inspect the heat range of the spark plugs. The plugs may be too hot.
    • Inspect the ignition timing. The timing may be over-advanced. Refer to Ignition System Description .
    • Verify the torque value of the spark plugs in order to ensure good plug to engine seat contact. Refer to .

Broken or Cracked Insulator Tips

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 Replacement .