USED <strong>SPARK</strong> <strong>PLUG</strong> ANALYSISAn effective way to diagnose an engine’s operating conditionsis to examine the used spark plugs for abnormal conditions. Ifall systems of a given cylinder are in proper working condition,the recommended spark plug will take on deposit, gap wearand insulator coloring characteristics which must be considerednormal. By indexing each plug to its respective cylinder andlocation during removal, the technician can later scrutinizeNORMALAppearance. Slight even wear on electrodes, very light deposits, acoloring of brown or greyish tan. Very clean, deposit-free firingends are common on LP and Natural gas applications.Indication. Correct spark plug for application; cylinder from whichremoved is “healthy.”WORN OUTAppearance. Very pronounced wear on electrodes, widenedgap space.Indication. Worn condition creates high voltage requirement andstresses the ignition system. Replace with equivalent heat range,perform routine ignition maintenance.BRIDGED GAPAppearance. A deposit lodged between the electrodes which “shortcircuits”the ignition voltage.Indication. Flaking off of combustion chamber deposits. Mayhappen after long periods of constant speed and load operationfollowed by cyclic accelerations.OIL ASH DEPOSITSAppearance. Light colored (generally grayish) crusty build-up onspark plug firing end. May vary in density from very soft and flakyto extremely hard and rock-like.Indication. Residual deposits from oil entering the combustionchamber in small amounts and being burned. These deposits,depending on oil additives, may be non-conductive and harmless.However, sufficient quantities of these deposits may build to maskthe entire firing end and affect initiation of combustion. Should thedeposits be conductive, the potential is great for core nose trackingand misfire.LANDFILL GAS DEPOSITSAppearance. Light colored (generally white) crusty build-up onspark plug firing end. May vary in density from very soft and flakyto extremely hard and rock-like.Indication. Residual deposit from an engine operated on gas froma landfill. This type of fuel is often referred to as "dirty" gas, thusthe naturally occurring deposits. These deposits may be non-conductiveand harmless, but sufficient quantities may build to maskthe entire firing end and interfere with the initiation of combustion.Should the deposits be conductive, the potential is great for corenose tracking and misfire.the plug under magnification to determine the possibility ofengine abnormalities.This procedure can be very useful if conducted as part ofregular maintenance inspections. The following series of photographsand explanations may serve as a guide to spark plugand engine analysis; conditions will vary from engine to engine.DETONATIONAppearance. Electrodes or insulator fractured/physically broken asif by mechanical means.Indication. Engine operated under abnormal combustion–detonation (uncontrolled burning or exploding of the fuel/aircharge). Generally due to excessive temperature from overheating,improper fuel/air ratios, over-advanced timing, coolingsystem blockages, insufficient engine tolerances, or improperbalance of fuel mixture and timing.PRE-IGNITIONAppearance. Overheated spark plug with electrodes and/orinsulator melted.Indication. Engine operated under abnormal combustion–pre-ignition (ignition prior to timed spark). Pre-ignition generallyresults from excessive temperatures created by “hot spots,”possibly carbon, sharp valve edge, gasket protruding into thechamber, or improper heat range plug.CORE NOSE TRACKINGAppearance. Ignition marks etched into the insulator core nose, orits deposits, that travel from the center electrode up the core noseto ground within the shell bore. These marks are often black incolor.Indication. Misfire may occur as the ignition spark has found analternate path to ground as opposed to the desired path of arcingfrom center electrode to the ground electrode. Tracking can becaused by any of the following conditions: Conductive deposits onthe insulator core nose, Too wide a spark gap, Too lean a fuel/airratio, Increased cylinder pressures at time of ignition.CARBON FOULEDAppearance. Soft, black, sooty deposits covering firing end.Indication. Possible problem in the fuel delivery system–overlyrich. Weak ignition system, flashover or secondary voltage, leak, orextremely low compression. More common to gasoline fueledauxiliary type engines.TORCHED SEATAppearance. Cutting torch effect in the thread and seat area of theplug shell.Indication. Plug was not seated properly, could not dissipate heatand allowed gases to blow by the threaded area.
USED <strong>SPARK</strong> <strong>PLUG</strong> ANALYSISANALYSE DE L’USURE DES BOUGIESANÁLISIS DE BUJÍAS USADASNormalDetonationUsure normaleNormalWorn OutDétonationDetonaciónPre-ignitionUsure totaleDesgastadaBridged GapPréallumageEncendido prematuroCore Nose TrackingÉcartement obstruéNo existe separación entre puntasOil Ash DepositsLandfill Gas DepositsCarbon FouledTraces de cheminement de l’isolantFormación de trayectorias en la narizTorched SeatDépôts résiduels d’huileDepósitos de cenizas aceitosasDépôts résiduels de gas d’enfouissementDepósitos de gas de vertederoCalaminageCubiertas de carbónSiège brûléAsiento quemado
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