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Spark Plug Failure Analysis

Appearance Possible Cause


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



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.





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.


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.


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.