Spark Plug Condition Chart Normal Mechanical Damage Oil Fouled May be caused by a foreign object that Too much oil is entering the combustion has accidentally entered the combus- Combustion deposits are slight chamber. This is often caused by piston tion chamber. When this condition is and not heavy enough to cause rings or cylinder walls that are badly discovered, check the other cylinders to any detrimental effect on engine worn. Oil may also be pulled into the prevent a recurrence, since it is possi- performance. Note the brown to chamber because of excessive clear- ble for a small object to "travel" from greyish tan color, and minimal ance in the valve stem guides. If the one cylinder to another where a large amount of electrode erosion which PCV valve is plugged or inoperative it degree of valve overlap exists. This clearly indicates the plug is in the can cause a build-up of crankcase pres- condition may also be due to improper correct heat range and has been sure which can force oil and oil vapors reach spark plugs that permit the piston operating in a "healthy" engine. past the rings and valve guides into the to touch or collide with the firing end. combustion chamber. Overheated Insulator Glazing Pre-Ignition A clean, white insulator firing tip and/or excessive electrode ero- Usually one or a combination of several sion indicates this spark plug con- Glazing appears as a yellowish, var- dition. This is often caused by over engine operating conditions are the nish-like color. This condition indicates prime causes of pre-ignition. It may origi- advanced ignition, timing, poor that spark plug temperatures have risen engine cooling system efficiency nate from glowing combustion chamber suddenly during a hard, fast accelera- deposits, hot spots in the combustion (scale, stoppages, low level), a tion period. As a result, normal combus- very lean air/fuel mixture, or a chamber due to poor control of engine tion deposits do not have an opportu- heat, cross-firing (electrical induction leaking intake manifold. When nity to "fluff-off" as they normally do. these conditions prevail, even a between spark plug wires), or the plug Instead, they melt to form a conductive heat range is too high for the engine or plug of the correct heat range will coating and misfire will occur. overheat. its operating conditions. Chart courtesy of Champion Spark Plugs Page 22 Spark Plug Condition Chart Gap Bridging Splash Fouled Detonation Rarely occurs in automotive en- gines, however, this condition is caused by similar conditions that Appears as "spotted" deposits on the firing tip of the insulator and often oc- produce splash fouling. Combus- This form of abnormal combustion has curs after a long delayed tune-up. By- tion deposits thrown loose may fractured the insulator core nose of the products of combustion may loosen lodge between the electrodes, plug. The explosion that occurs in this suddenly when normal combustion tem- causing a dead short and misfire. situation applies extreme pressures on peratures are restored. During hard Fluffy materials that accumulate internal engine components. Prime acceleration these materials shed from on the side electrode may melt to causes include ignition time advanced too the piston crown or valve heads, and bridge the gap when the engine far, lean air/fuel mixtures, and insufficient are thrown against the hot insulator sur- is suddenly put under a heavy octane rating of the gasoline. load. face. Ash Fouled Carbon Fouled Worn Soft, black, sooty deposits easily iden- This plug has served its useful life and tify this plug condition. This is most of- should be replaced. The voltage required A build-up of combustion depos- ten caused by an over-rich, air/fuel mix- to fire the plug has approximately doubled its stemming primarily from the ture. and will continue to increase with addi- burning of oil and/or fuel additives Check for a sticking choke, clogged air tional miles of travel. Even higher voltage during normal combustion ... nor- cleaner, or a carburetor problem - float requirements, as much as 100% above mally non-conductive. When level high, defective needle or seat, etc. normal, may occur when the engine is heavier deposits are allowed to This may also be attributed to weak quickly accelerated. Poor engine perform- accumulate over a longer mile- ignition voltage, an inoperative preheat- ance and a loss in fuel economy are traits age period, they can "mask" the ing system (carburetor intake air), or of a worn spark extremely low cylinder compression. spark, resulting in a plug misfire condition. Chart courtesy of Champion Spark Plugs Page 23 .