spark plug diagnosis

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The Car Care Guide
Keeping Your Car Healthy

Spark Plug Fault Diagnosis

This section identifies some common faults associated with spark plug wear, incorrect applications and general problems with engine running conditions. The condition of the firing end of a spark plug is a good indicator of the general health of an engine.

	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.