How To Read A Spark Plug I

The following information is extracted from an article originally posted on the now non-working Cooper Auto Site. The name of the full article was "A Complete Guide To Spark Plug Performance" (Copyright© 1998 Cooper Automotive) based on information from Champion.

Normal Firing End

A grayish tan to while color indicates the correct heat range spark plug is in use, the fuel and ignition systems are in good shape and overall engine mechanical condition is good. Replace with Champion plugs of the same heatrange. Refer to the owner's manual for recommended spark plug change interval.

Worn Out

Worn or rounded center and/or ground electrodes indicate excessive wear and can cause misfire during acceleration, hard starting or reduced fuel economy and damage to other secondary ignition components e.g., spark plug wires, coils and distributor cap with continuous use.

When a plug has this type of appearance, it has simply exceeded its useful life. Replace with new Champion spark plugs of the same heat range and design.

Mechanical Damage

Bent and/or broken electrodes and core nose indicate mechanical damage caused by foreign objects in the combustion chamber or improper reach spark plugs. Remove the foreign object from the engine and check the Champion catalog for proper spark plug application.


In cases of light detonation, small black or gray spots will be noticed on the core nose of the spark plug. In cases of severe detonation, insulators may be cracked and/or chipped. The same high pressure waves created during detonation can break spark plugs, damage intake valves and break pistons.

Make sure that the correct octane fuel is being used, assure proper operation of emission and computer systems (paying special attention to the EGR system) and assure the correct heat range of spark plug is being used.

Ash Deposits

Light brown deposits encrusted on the ground and/or center electrodes indicate ash deposits. This situation is caused by oil and/or fuel additives. When the deposits are found on only one side of the spark plug core nose, it is usually considered to be a problem with the cylinder head (valve stem seals or valve guides). When they are found on both sides of the spark plug, it is often considered to be a problem sealing at the piston rings. This condition can mask the spark and, in some cases, contribute to misfire.

Check for worn valve guides and valve stem seals and/or piston rings. The spark plug is the correct heat range and was a victim of the engine's condition, not the cause of it. Champion does not recommend the use of fuel additives which leave deposits on the core nose of the spark plug.

Oil Fouled

Symptoms of oil fouling include black oily coating caused by poor oil control. This situation is more severe than what is seen with the ash-fouled spark plugs and usually represents advanced engine wear. When the oil enters the combustion chamber and covers the core nose of the spark plug, the spark no longer arcs across the gap. Rather it takes the easier path to ground by tracking down the oil on the core nose. This results in a complete cylinder misfire condition.

Check for worn valve guides, valve stem seals and/or piston rings, In some engines, a defective PCV valve can contribute to this condition. Replacing the spark plug may help for a short time, but the new plug will soon foul.


Chalky white insulator with little or no coloration, accelerated electrode wear and possibly blistered or pitted electrodes are indications that a plug has overheated. The shell may also be discolored from light gray to a dark blue, almost black.

Check for the correct heat range spark plug. Verify that ignition liming and air fuel mixtures are adequate and that all emission systems are operationaf. Inoperative or malfunctioning EGR systems are often the cause of an overheated condition. Engine overheating and restricted exhaust systems can also cause this condition.

Initial And Sustained Pre-Ignition

Signs of the spark plug being hot or blistered and/or melted center and ground electrodes are indications of initial preignition. Check that the correct heat range spark plug is being used, assure ignition timing and air fuel mixture are appropriate, assure entire ignition system is functional and check its specifications. Pay special attention to the EGR system and/or knock sensors. Routing of spark plug wires on some engines can contribute to cross induction which will lead to pre-ignition. Excessive carbon deposits in the combustion chamber may contribute as well.

Melted center and/or ground electrodes and/or a melted insulator are symptoms of sustained pre-ignition. See initial preignition, description above. Also expect to find damage to the pistons and/or exhaust valves.

Splash Deposits

Small islands of contaminants on the insulator indicate splash deposits. Replace with new Champion plugs of the correct heat range. The use of fuel additives, carburetor and choke cleaners or other aggressive solvents before installing new plugs is the most common cause of this condition.

Carbon Fouled

Soft, black, sooty, dry-looking deposits indicate a rich air fuel mixture, weak ignition or wrong heat range spark plug (too cold). These carbon-based deposits are conductive, much like oil fouling, and will allow the voltage coming out of the center electrode of the spark plug to track down the core nose rather than jumping the gap. This will result in an engine misfire and further aggravate the carbon fouled condition.

Check for correct plug heat range. On fuel injected engines, check for sticking injectors, malfunctioning cold start valves and/or circuits. Also check for correct fuel pressure specifications. On computer controlled vehicles, the "limp home" computer mode will always result in a rich condition. Therefore, it is imperative that you check the operation and condition of the on-board computer system. On carbureted vehicles, check choke and choke pulloff, high float level, and needle and seat condition. On all engines, severe vacuum leaks can decrease manifold vacuum, resulting in a rich condition. Weak and/or damaged secondary ignition systems will fail to spark across the gap lowering combustion chamber temperatures and promoting carbon deposits. This condition could also result from continuous low speed driving or poor cylinder compression.

Fuel Additives

Red to purple deposits on one side of the core nose are an indication of a fuel additive. While many of these deposits are non-conductive and do not contribute to lack of performance, some fuel additives contain octane boosters that leave conductive deposits on the core nose. Care should be taken to select fuel additives which are compatible with ignition systems and do not contain conductive materials such as octane boosters.