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Spark Plug Diagnosis Chart




Symptoms: Brown to grayish-tan and slight electrode wear. Correct heat range for engine ad operating conditions.

Recommendations: When new spark plugs are installed, replace with plugs of the same heat range.


Symptoms: Rounded electrodes with a small amount of a deposit on the firing end. Normal Color. Causes hard starting in damp or cold weather and poor fuel economy

Recommendations: Plugs have been left in the engine too long. Replace with new plugs of the same heat range. Follow the recommended maintenance schedule.


Symptoms: Dry sooty deposits indicates a rich mixture or weak ignition. Causes misfiring, hard starting, and hesitation.

Recommendations: Make sure the plug has the correct heat Range, Check for clogged air filter or problem in the fuel system or engine management system. Also check for ignition system problems.


Symptoms: Light brown deposits encrusted on the side or center electrodes or both. Derived from oil ad/or fuel. Excessive amount may mask the spark, Causing misfiring and hesitation during acceleration.

Recommendations: If excessive deposits accumulate over a short time or mileages, install new valve guide seals to prevent seep-age of oil into the combustion chambers. Also try changing gasoline brands.


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.

Recommendations: Correct thr mechanical condition with necessary repairs and install new plugs.


Symptoms: Combustion deposits lodge between the electrodes, Heavy deposits accumulate ad bridge the electrode gap. The plug ceases to fire, resulting in a dead cylinder.

Recommendations: Locate the faulty plug and remove the deposits from between the electrodes.


Symptoms: Blistered, white insulator, eroded electrode and absence of deposits. Result in shortened plug life.

Recommendations: Check for the correct plug heat range, over advanced ignition timing, lean fuel mixture, intake manifold vacuum leaks, sticking valves and insufficient engine cooling.


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.

Recommendations: Check for the correct plug heat range, over advance ignition timing, lean fuel mixture, insufficient engine cooling and lack of lubrication.


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.

Recommendations: Install new plugs. Consider using colder plug if driving habits warrant.


Symptoms: Insulator may be cracked or chipped, improper gap setting techniques can also result in a fractured insulator tip. Can lead to piston damage

Recommendations: Make sure the fuel anti-knock values meet engine requirements. Use care when setting the gaps on new plugs. Avoid lugging the engine.


Symptoms: May be causes 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.

Recommendations: Repair the mechanical damage. Remove the foreign object from the engine and/or install the correct reach plug.


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