Normal Spark Plug
Correct heat range of spark plug is being used.
Appearance- Grayish-tan to white color.
Normal with Red Coating
Coloration is from the use of additives in unleaded gas.
Appearance- Pinkish red color on the insulator.
Fuel Fouled
Indicates the cylinder from which the spark plug came from is not using all the fuel supplied to it. Possible to cold of heat range on plug or too rich fuel mixture.
Appearance- Firing tip is damp with gasoline and insulator is charcoal black.
Detonation
Caused by low octane fuel or over advanced timing.
Appearance- Insulator is usually cracked or chipped or broken.
Worn Out
Spark Plug used beyond its intended life.
Appearance - Center and ground electrodes are eroded , and excessively worn away.
Glazing
Spark plug is operating too hot at high speeds. Replace with colder plug.
Appearance- Ceramic insulator tip appears to have a melted, glazed coating.
Carbon Foulded
Spark plug heat range is too cold and/or caused by extensive low-speed; short distance driving.
Appearance- black sooty coating on firing end.
Sustained Preignition
Check for correct application of spark plug (heat range to hot, wrong spark plug for engine, over advanced timing or lean fuel mixture.
Appearance- Melted center and ground electrodes
.
Ash Deposits
Caused by the use of leaded fuel , fuel additives and /or oil additives. Check for worn rings and/or valve guides
Appearance- center electrode and ground electrode and or ceramic insulator tip tan in colored deposits.
Oil Foulded
Caused by presence of oil in the combustion chamber. Worn rings, valve guides or valve seals.
Appearance - center electrode and or ceramic insulator coated black, oil substance.
Mechanical Damage
locate and remove foreign object from inside of cylinder. Check for improper spark plug reach.
Lead Fouled
Occurrence is from use of leaded fuel or additives containing lead.
Appearance- ceramic insulator tip is coated with a brownish-yellow glazed coating.