NORMAL

SYMPTOM:Brown to grayish color and slight electrode wear. Correct heat range for engine & driving conditions.

RECOMMENDATION:Replace with same heat range Spark Plug.

WORN

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

RECOMMENDATION: Plugs have been left in engine too long. Replace with new plugs of the same heat range.

ASH DEPOSITS

SYMPTOM: Light brown depostis encrusted on the side or center electrode. Derived from oil or fuel additives. This may cause mis-fire and hesitation during accelleration.

RECOMMENDATION: Replace valve oil seals if plugs get depostis in 500 miles or less, Switch to different brand of fuel.

CARBON DEPOSITS

SYMPTOM: Dry sooty depostis indicate a rich fuel mixture or weak ingnition. Hard starts, misfiring, and hesitation.

RECOMMENDATION: Possible incorrect heat range. Clogged air filter, incorrect fuel delivery. Try a hotter heat range plug, check filters, check fuel system.

OIL DEPOSITS

SYMPTOM: Wey oily electrodes caused by poor oil control. Hard starting, hesitation, and misfiring.

RECOMMENDATION: Correct mechanical problems.

GAP BRIDGING

SYMPTOM: Combustion deposits lodge between electrodes.Heavy deposits accumulate and bridge the gap.

RECOMMENDATIONS: Locate faulty plug, clean and re-install. If bridging persists then mechanical repairs may be in order. Service heads. 

TOO HOT!

SYMPTOMS:Blistered white insulator, eroded electrode and absence of deposits. Possible deformed electrode.

RECOMMENDATIONS: Lower heat range, check ignition timing, check for air leaks. 

PREIGNITION

SYMPTOMS: Melted electrodes. Insulators may be white, but dirty due to misfiring. Can lead to major engine damage.

RECOMMENDATION: Change heat range, check ignition timing, check fuel delivery, check cooling system.

HIGH SPEED GLAZING

SYMPTOM: Insulator has yellowish color, and glossy. Caused by sudden combustion chamber temperature rise.

RECOMMENDATION: Lower heat range, go easy on the hotrodding, or improve cooling system.

DETONATION

SYMPTOM: Insulator may be cracked or chipped. Caused by bad gap setting technique, or bad piston to spark plug clearance.

RECOMMENDATION: Replace plugs with proper gap and mfg. spark plug recommendation. Check plug to piston clearance. Avoid lugging the engine