If you take a good look at the condition of any used spark plug, you will be able to tell a lot about the condition of the engine it came from. Spark plugs wear in predictable patterns. The state and colour of the spark plug can show how efficiently the engine is running, or if there are any imminent problems. The condition of the spark plug can be compared to the pictures shown below for further analysis.
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Appearance |
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Possible cause/resolution |
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Normal
Light brown, tan or grey firing end.
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A good indicator that the plug is functioning correctly and general engine conditions are good.
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Dry fouling
Fouling, dry - matt black, and sooty. Carbon deposits build up when the plug fails to fire correctly and burn them off.
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Air/fuel mixture too rich, carb float height needs adjusting, float sticks open, choke stuck on, electrical problem, extended idling, plug heat range too cold, dirty air filter.
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Wet fouling
Fouling, wet - glossy black, and sticky.
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Caused by oil leaking past the piston rings or valve guides and burning in the cylinder. Will have blue smoke coming out of exhaust. If new plug is installed it will soon foul and stop working. |
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Overheating
When overheating occurs, deposits which have accumulated on the insulator tip may melt and give the tip a glazed appearance.
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Possible causes are over advanced ignition timing, air/fuel mixture too lean, water or oil level too low, plugs not fitted (tightened) correctly, plug heat range too hot. |
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Deposits
Insualtor nose and electrodes encrusted with a build of deposits - usually off-white in colour.
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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. |
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Lead fouling
Lead deposits on the insulator nose. These are usually a yellowish brown in colour.
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Lead content of petrol used is too high. Try petrol with a lower lead content.
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Breakage
Physical damage to the insulator nose.
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Usually caused by abnormal thermal expansion in the combustion chamber. Maybe thermal heating or cooling shock. Causes as for overheating above. |
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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. |
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Plugs are at the end of servicable life. Replace plugs as a set.
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Abnormal erosion
This is accelerated growth of the gap.
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due to the effects of corrosion, oxidation and reaction with the lead in petrol.
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Melting
The electrode surface will probably appear lustrous and uneven.
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Due to excessively high temperatures in the combustion chamber. Causes as for overheating above. |
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Erosion, Corrosion, and Oxidisation
The surfaces of the electrodes are rough, in extreme circumstances the electrode material will have oxidised to the point of turning green. |
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Possibly due to age, vehicle standing for a long time without use.
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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.
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This is due to chemical reaction between the nickel alloy electrodes and the lead compounds in petrol.
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