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Reading a Spark Plug

NASCAR engine tuners love spark plugs they can "read". The discoloration on the insulator after a few laps helps them understand how the electrical elements of the engine are performing.
Robert Reeves

Using the DENSO plugs during qualifying, we gained a 10th of a second routinely...
Not all plugs allow you to "read" them. The corona, a brown deposit on the insulator, is the glow that appears above the spark plug housing around the base of the insulator. The appearance of a corona is not detrimental to the operation of the spark plug. Contrary to common belief, this condition is caused by electrical stress in the air adjacent to the insulator. Often, the corona stain is mistaken for spark plug gas leakage, but again, this does not affect spark plug performance.

The insulator is ceramic and can be broken, although this is a rare occurrence. Typically, the most common ways the insulator becomes cracked is when the installation is done at the wrong angle, the incorrect torque is used or the removal is forced. (See Installation Guide)

Our primary concern is that you use the appropriate tools and torque during installation. When the insulator is cracked during installation, this can cause a short circuit which will result in irregular idling and misfires during acceleration. (See Common Engine Problems)


Diagnosis of a Used Spark Plug

If you're seeing any of the following, you probably don't have the manufacturer's recommended spark plug installed. But the symptoms are separate from the causes and all the components in an ignition system, no matter how small, will leave some signs on the spark plugs over time if they're not in harmony. This guide can provide insight to the engine's performance.
Normal Normal
Light grey or tan deposits and slight electrode erosion


Carbon Fouling Carbon Fouling
Dry, soft black carbon on the insulator and electrodes.

Symptoms:
  • Poor starting
  • Misfiring
  • Faulty acceleration


  • Causes:
  • Faulty choke - over rich air/fuel mixture
  • Delayed ignition timing
  • Bad ignition leads
  • Plug heat range too cold


  • Pre-Ignition Pre-Ignition
    A melted or burned center and/or ground electrode, blistered insulator and aluminum or other metallic deposits on the insulator

    Symptoms:
  • Loss of power causing engine damage
  • Pre-ignition occurs when combustion begins before the timed spark occurs.


  • Causes:
  • Plug insufficiently tightened
  • Engine insufficiently cooled
  • Ignition timing too advanced
  • Plug heat range too hot


  • Over Heating Over Heating
    An extremely white insulator with small black deposits and premature electrode erosion.

    Symptoms:
  • Loss of power at high-speed or during heavy load


  • Causes:
  • Plug insufficiently tightened
  • Engine insufficiently cooled
  • Ignition timing too advanced
  • Plug heat range too hot


  • Mechanical Damage Mechanical Damage
    Bent electrode and a broken insulator, dents often present on electrode

    Symptoms:
  • Misfiring


  • Causes:
  • Plug nose is too long for engine head
  • Foreign object (bolt/nut) in combustion chamber


  • Oil Fouling Oil Fouling
    Wet, oily black deposits on the insulator and electrodes

    Symptoms:
  • Poor starting
  • Misfiring


  • Causes:
  • Wrong piston rings, cylinders, and valve guides
  • Fuel mixture oil content too high (two-stroke engines)


  • Broken Insulator Broken Insulator
    Insulator is cracked or split

    Symptoms:
  • Misfiring


  • Causes:
  • Severe detonation
  • Incorrect tool/torque applied during installation or removal
  • Careless gap setting


  • Torched Seat Torched Seat
    Melted in the thread and seat area of the plug housing

    Symptoms:
  • Loss of power causing engine damage


  • Causes:
  • Plug insufficiently tightened





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