Diagnosis of a Used Spark Plug
If you’re seeing any of the following, you probably don’t have the manufacturers recommended spark plug installed. But the symptoms are separate from the cause 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 other areas of the engine’s performance.
 | NORMAL Light grey or tan deposits and slight electrode erosion. |
 | 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. |
 | OVER HEATING An extremely white insulator with small black deposits and premature electrode erosion. Symptoms: Loss of power at high-speeds or during heavy load. Causes: Plug insufficiently tightened. Engine insufficiently cooled. Ignition timing too advanced. Plug heat range too hot. |
 | 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. |
 | PRE-IGNITION A melted or burned centre and/or ground electrode, blistered insulator and aluminium 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. |
 | OIL FOULING Wet, oily black deposits on the insulator and electrodes. Symptoms: Poor starting, misfiring. Causes: Wrong piston rings, cylinders and valve guides. New or recently overhauled engines. Fuel mixture oil content too high. |
 | BROKEN INSULATOR Insulator is cracked or split Symptoms: Misfiring
Causes:Severe detonation. Incorrect tool/torque applied during installation or removal. |
 | TORCHED SEAT Melted in the thread and seat area of the plug housing Symptoms: Loss of power causing engine damage Causes: Plug insufficiently tightened |