Spark Plug: Testing and Inspection

SPARK PLUG CONDITION
Remove the spark plugs and examine them for burned electrodes and fouled, cracked or broken porcelain insulators. Keep plugs arranged in the order in which they were removed from the engine. An isolated plug displaying an abnormal condition indicates that a problem exists in the corresponding cylinder.

Normal Operation and Cold (Carbon) Fouling:

Normal Operating Conditions
The few deposits present will be probably light tan or slightly gray in color with most grades of commercial gasoline. There will not be evidence of electrode burning. Gap growth will not average more than approximately 0.025 mm (0.001 in) per 1600 km (1000 miles) of operation for non platinum spark plugs. Non-platinum spark plugs that have normal wear can usually be cleaned, and then reinstalled.

CAUTION: Never attempt to file the electrodes or use a wire brush for cleaning platinum spark plugs. This would damage the platinum pads which would shorten spark plug life.

Some fuel refiners in several areas of the United States have introduced a manganese additive (MMT) for unleaded fuel. During combustion, fuel with MMT may coat the entire tip of the spark plug with a rust colored deposit. The rust color deposits can be misdiagnosed as being caused by coolant in the combustion chamber. Spark plug performance may be affected by MMT deposits.

Cold Fouling (Carbon Fouling)
Cold fouling is sometimes referred to as carbon fouling because the deposits that cause cold fouling are basically carbon. A dry, black deposit on one or two plugs in a set may be caused by sticking valves or misfire conditions. Cold (carbon) fouling of the entire set may be caused by a clogged air cleaner.

Cold fouling is normal after short operating periods. The spark plugs do not reach a high enough operating temperature during short operating periods. Replace carbon fouled plugs with new spark plugs.

Fuel Fouling
A spark plug that is coated with excessive wet fuel is called fuel fouled. This condition is normally observed during hard start periods. Clean fuel fouled spark plugs with compressed air and reinstall them in the engine.

Oil Fouling
A spark plug that is coated with excessive wet oil is oil fouled. In older engines, wet fouling can be caused by worn rings or excessive cylinder wear. Break-in fouling of new engines may occur before normal oil control is achieved. Replace oil fouled spark plugs with new ones.

Oil or Ash Encrusted:

Oil Or Ash Encrusted
If one or more plugs are oil or ash encrusted, evaluate the engine for the cause of oil entering the combustion chambers. Sometimes fuel additives can cause ash encrustation on an entire set of spark plugs. Ash encrusted spark plugs can be cleaned and reused.

High Speed Miss
When replacing spark plugs because of a high speed miss condition; wide open throttle operation should be avoided for approximately 80 km (50 miles) after installation of new plugs. This will allow deposit shifting in the combustion chamber to take place gradually and avoid plug destroying splash fouling shortly after the plug change.

Electrode Gap Bridging:

Electrode Gap Bridging
Loose deposits in the combustion chamber can cause electrode gap bridging. The deposits accumulate on the spark plugs during continuous stop-and-go driving. When the engine is suddenly subjected to a high torque load, the deposits partially liquefy and bridge the gap between the electrodes. This short circuits the electrodes. Spark plugs with electrode gap bridging can be cleaned and reused.

Scavenger Deposits:

Scavenger Deposits
Fuel scavenger deposits may be either white or yellow. They may appear to be harmful, but are a normal condition caused by chemical additives in certain fuels. These additives are designed to change the chemical nature of deposits and decrease spark plug misfire tendencies. Notice that accumulation on the ground electrode and shell area may be heavy but the deposits are easily removed. Spark plugs with scavenger deposits can be considered normal in condition, cleaned and reused.

Chipped Electrode Insulator:

Chipped Electrode Insulator
A chipped electrode insulator usually results from bending the center electrode while adjusting the spark plug electrode gap. Under certain conditions, severe detonation also can separate the insulator from the center electrode. Spark plugs with chipped electrode insulators must be replaced.

Preignition Damage:

Preignition Damage
Excessive combustion chamber temperature can cause preignition damage. First, the center electrode dissolves and the ground electrode dissolves somewhat later. Insulators appear relatively deposit free. Determine if the spark plugs are the correct type, as specified on the VECI label, or if other operating conditions are causing engine overheating.

Spark Plug Overheating:

Spark Plug Overheating
Overheating is indicated by a white or gray center electrode insulator that also appears blistered. The increase in electrode gap will be considerably in excess of 0.001 in per 1000 miles of operation. This suggests that a plug with a cooler heat range rating should be used. Over advanced ignition timing, detonation and cooling system malfunctions also can cause spark plug overheating.