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SPARK PLUG CONDITION

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Normal

Light tan or gray deposits , almost brown with LPG or natural-gas fuel . Yellow or tan deposits may indicate leaded fuel . Slight electrode wear . Plug running at correct temperature in a 'healty' engine .

Worn

Electrode worn away . This may cause firing voltage to double (even higher on acceleration) , poor engine performance , and loss fuel economy . Plug has served its useful life .

Preignition

Melting of center electrode , later melting of ground electrode and insulator .

Detonation

Unsulator nose has cracked and broken as a result of shock waves from this abnormal combustion .

Mechanical damage

Foreign object in combushtion camber or on piston has struck ground electrode , forcing it into center electrode which has bent and brocken off insulator nose . Valve overlap may allow object to travel from one cylinder to another .

Gas bridged

Causes by conditions similar to splash fouling . Combushtion deposits thrown loose lodge between electrodes , causing short and misfire . Fluffy materials that accumulate on side electrode melt and bridge gap when a sudden heavy load applied to engine .

Carbon fouled

Soft , soofty carbon deposits with a dry , black appearance .

Oil fouled

Wet , oily coating , plug shorted by excessive oil entering combushtion chamber .

Splashed fouled

May occur after overdue tuneup . Deposits caused by misfiring and low-power operation break loose when tuneup restores normal combushtion temperature . Theese deposits are then thrown against the hot insulator .

Ash fouled

Buildup of deposits primarily from burning of oil and fuel additives during normal combushtion . Ash deposits usually nonconductive , plug misfire .

Overheated

Blistered , white or gray insulator nose . Rapid electrode-gap wear .

Glazed

Insulator has shiny conductive coating that may cause misfire at high speed . Results from sudden increase in temperature during hard accelaration . Then normal metallic deposits melt before they shed from the insulator .