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Normal:
Brown to grayish-tan color and slight electrode wear. Correct heat range for engine and operating conditions. |
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Worn:
Rounded electrodes with a small amount of deposits on the firing end. Normal color. Causes hard starting in damp or cold weather and poor fuel economy. The voltage required to fire the plug has approximately doubled and will continue to increase with additional miles of travel. Replace with new plugs. |
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Overheating:
Blistered, white insulator, eroded electrode and absence of deposits. Results in shortened plug life. This is often caused by over advanced ignition timing, poor engine cooling system efficiency (scale, stoppages, low coolant level), a very lean air/fuel mixture, or a leaking intake manifold. When these conditions prevail, even a plug of the correct heat range will overheat. |
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Oil Fouled:
Oily coating caused by poor oil control. Oil is leaking past worn valve guides or piston rings into the combustion chamber. Causes hard starting, misfiring and hesitation. |
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Pre-Ignition:
Melted electrodes. Insulators are white, but may be dirty due to misfiring or flying debris in the combustion chamber. Can lead to engine damage. Check for the correct plug heat range, over advanced ignition timing, lean fuel mixture, insufficient engine cooling and lack of lubrication. |
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Detonation:
Insulators may be cracked or chipped. Can lead to piston damage. The explosion that occurs in this situation apples extreme pressures on internal engine components. Prime causes include ignition time advanced too far, lean air/fuel mixtures, and insufficient octane rating of the gasoline. |
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Mechanical Damage:
May be caused by a foreign object in the combustion chamber or the piston striking an incorrect reach (too long) plug. Causes a dead cylinder and could result in piston damage. Find out what caused the damage and repair it! When this condition is discovered, check the other cylinders to prevent a recurrence, since it is possible for a small object to "travel" from one cylinder to another where a large degree of valve overlap exists. |
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Gap Bridging:
Combustion deposits lodge between the electrodes. Heavy deposits accumulate and bridge the electrode gap. The plug ceases fire, resulting in a dead cylinder. Combustion deposits thrown loose may lodge between the electrodes, causing a dead short and misfire. Fluffy materials that accumulate on the side electrode may melt to bridge the gap when the engine is suddenly put under a heavy load. |
