Normal
Brown to grayish-tan color and slight electrode wear. Correct heat range for engine and operating conditions.
Worn
Symptoms: Rounded electrodes with small amount of deposits on the firing end. Normal color. Causes hard starting in damp or cold weather and poor fuel economy. Plugs have been left in the engine too long. Replace with new plugs of the same heat range.
Carbon Deposits
Symptoms: Dry, sooty deposits indicating a rich mixture or weak ignition. Causes misfiring, hard starting and hesitation. Make sure the plug has the correct heat range. Check for a clogged air filter or problem in the fuel system. Also check for ignition system problems.
Ash Deposits
Symptoms: Light brown deposits encrusted on the side or center electrodes or both. Derived from oil and/or fuel additives. Excessive amounts may mask the spark, causing misfiring and hesitation during acceleration. If excessive deposits accumulate over a short time or low mileage, install new valve guide seals to prevent seepage of oil into the combustion chambers. Also try changing gasoline brands.
Oil Deposits
Symptoms: 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. Blue smoke will be seen coming from exhaust. Correct the mechanical problem causing the condition and install new plugs.
Gap Bridging
Symptoms: Combustion deposits lodge between the electrodes. Heavy deposits accumulate and bridge the electrode gap. The plug ceases to fire, resulting in a dead cylinder. Remove the deposits from between the electrodes.
Too Hot
Symptoms: Blistered, white insulator, eroded electrode and absence of deposits. Results in shortened plug life. Check for the correct plug heat range, over-advanced ignition timing, lean fuel mixture, intake manifold vacuum leaks, sticking valves and insufficient engine cooling.
Preignition
Symptoms: 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.
High Speed Glazing
Symptoms: Insulator has yellowish, glazed appearance. Indicates that combustion chamber temperatures have risen suddenly during hard acceleration. Normal deposits melt to form a conductive coating. Causes misfiring at high speeds. Install new plugs. Consider using a colder plug if driving habits warrant.
Detonation
Symptoms: Insulators may be cracked or chipped. Improper gap setting techniques can also result in a fractured insulator tip. Can lead to piston damage. Make sure the fuel octane is high enough. Use care when setting the gaps on new plugs. Avoid lugging the engine.
Mechanical Damage
Symptoms: May be caused by a foreign object in the combustion chamber or the piston striking an plug that is too long. Causes a dead cylinder and could result in piston damage. Repair the damage and correct the cause.
Coolant Leak/Head Gasket
Symptoms: Coolant is entering the cylinder, either through a leaking/failed head gasket, or a cracked block. The coolant "steam cleans" the plug, so the plug appears clean and shiny.
Mixture
Incorrect carburetor mixture can also be seen on spark plugs:
- Incorrect Mixture Results
Some more examples:
- Spark plug examples
