|
Appearance |
Possible cause/resolution |
|
Normal
Light brown, tan or grey firing end. |
A good indicator that the plug is functioning correctly and general engine conditions are good. |
|
Dry and wet fouling
Fouling, either dry (top - matt black, sooty) or wet (bottom - gloss black, sticky), |
Can be caused by many different conditions. Carbon deposits build up when the plug fails to fire correctly and burn them off. Air/fuel mixture too rich, choke stuck on, electrical problem, extended periods of low speed driving, plug heat range too cold. All should be investigated |
|
Overheating
When overheating occurs, deposits which have accumulated on the insulator tip may melt and give the tip a glazed appearance |
Possible causes are overadvanced ignition timing, air/fuel mixture too lean, water or oil level too low, plugs not fitted (tightened) correctly, plug heat range too hot. |
|
Deposits
Insualtor nose and electrodes encrusted with a build of deposits - usually off white in colour.
|
This is often caused by oil leakage through the piston rings or valve seals. Could be due to the wrong viscosity of oil being used. |
|
Lead fouling
Lead deposits on the insulator nose. These are usually a yellowish brown in colour. |
Lead content of petrol used is too high. Try petrol with a lower lead content. |
|
Breakage
Physical damage to the insulator nose. |
Usually caused by abnormal thermal expansion in the combustion chamber. Maybe thermal heating or cooling shock. Causes as for overheating above. |
|
Normal life
Growth of the plug gap during a plug's working life is normal. However, the increased gap will mean the spark is less efficient and hence fuel is wasted and strain is put on the ignition system. |
Plugs are at the end of servicable life. Replace plugs as a set. |
|
Abnormal erosion
This is accelerated growth of the gap |
due to the effects of corrosion, oxidation and reaction with the lead in petrol. |
|
Melting
The electrode surface will probably appear lustrous and uneven. |
Due to excessively high temperatures in the combustion chamber. Causes as for overheating above. |
|
Erosion, Corrosion, Oxidisation
The surfaces of the electrodes are rough, in extreme circumstances the electrode material will have oxidised to the point of turning green |
Possibly due to age, vehicle standing for a long time without use. |
|
Lead Erosion
The ground electrode will appear worn away, the central electrode will appear chipped and the insulator nose will take on a yellowish brown. |
This is due to chemical reaction between the nickel alloy electrodes and the lead compounds in petrol. |
