Common Spark Plug Condition

NORMAL

Symptoms: Brown
to grayish-tan color and slight electrode wear. Correct heat range for engine and
operation conditions

Recommendation: When
new spark plugs are installed, replace with plugs of the same heat range.

OVER HEATED

Symptoms: Blistered, white
insulator, eroded electrode and absence of deposits. Results in shortened plug life

Recommendation:
Check for the correct plug heat range, over advanced ignition timing, lean
fuel mixture, intake manifold vacuum leaks, sticking valves and insufficient engine
cooling.




WORN
Symptoms:
Rounded electrodes with a 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 the engine tool long. Replace with new plugs of the
same heat range. Follow the recommended maintenance schedule.



CARBON DEPOSITS
Symptoms: Dry
sooty deposits indicate a rich mixture or weak ignition. Causes misfiring, hard starting
and hesitation

Recommendation:
Make sure the plug has the correct heat range. Check for a clogged air
filter or problem in the fuel system or engine management system. Also check for ignition
system problems




PRE IGNITION
Symptoms:
Melted electrodes. Insulators are white but may be dirty due to misfiring or
flying debris in the combustion chamber. Can lead to engine damage.

Recommendation:
Check for the correct plug heat range, over advanced ignition timing, lean
fuel mixture, insuficient engine cooling and lack of lubrication.



ASH DEPOSIT
Symptoms: Light
brown deposits encrusted on the side or center electrodes or both. Derived from all and/or
fuel additives. Excessive amounts may mask the spark, causing misfiring and hesitation
during acceleration.

Recommendation: If excessive deposits accumulate over a short time or
low mileage, install new valve guide seals to prevent seepage of oil into the combustion
chambers. Also try changing gasoline brands.

HIGH SPEED GLAZING
Symptoms:
Insulator has yellowish, glazed appearance. Indicates that combustion
chamber temperatures have risen suddenly during hard acceleration. Normal deposits melt to
form a conductive coating. Causes misfiring at high speeds.

Recommendation: Install new plugs. Confider using a colder plug if
driving habits warrant



OIL DEPOSITS
Symptoms: Oily
coating caused by poor oil control. Oil is leaking past worn valve guides or piston rings
into the combustion chamber. Causes hard starting, misfiring and hesitation.

Recommendation:
Correct the mechanical condition with necessary repairs and install new
plugs.




GAP BRIDGING
Symptoms:
Combustion deposits lodge between the electrodes. Heavy deposits accumulate
and bridge the electrode gap. The plug ceases to fire, resulting in a dead cylinder.

Recommendation:
Locate the faulty plug and remove the deposits from between the electrodes.




DETONATION

Symptoms:
Insulators may be cracked or chipped. Improper gap setting techniques can
also result in a fractured insulator tip. Can lead to piston damage.

Recommendation:
Make sure the fuel anti-knock values meet engine requirements. Use care when
setting the gaps on new plugs. Avoid lugging the engine

MECHANICAL DAMAGE

Symptoms: May
be caused by a foreign object in the combustion chamber or the piston striking an
incorrect reach (too long) plug.
Causes a dead cylinder and could result in piston damage.

Recommendation:
Repair the mechanical damage. Remove the foreign object from the engine
and/or install the correct reach plug.