SPARK PLUG FIRING ANALYSIS

The appearance of the electrodes and insulator of the spark plug - the " spark plug face " - provides information on the operating performance of the spark plug as well as on the mixture and combustion in the engine. The assessment of the face of the spark plug is, therefore an important part of engine diagnosis. By utilising the spark plug condition in conjunction with oscilloscope and emission analyser readings, diagnosis of particular engine faults are further enhanced and thus correct repairs can be carried out.

EXAMPLES OF SPARK PLUG FIRING END ANALYSIS

	NORMAL : The spark plug face shows the insulator nose which has slight combustion deposits. The deposits are not heavy enough to cause any detrimental effect to engine performance. The plug face colour can range from brown to greyish white. There should be minimal electrode erosion which indicates correct heat range and thus the plug has been functioning under good engine conditions.
	CARBON FOULED : The spark plug face shows dry, soft, black sooty deposits on the insulator nose and electrodes. An accumulation of this sooty carbon causes the insulation between the center and ground electrodes to deteriorate and an electrical leakage path is formed by the carbon resulting in a misfire. An overly rich fuel mixture, prolonged low speed idling or driving, spark plug heat range too cold or an ignition system fault are some causes for this plug condition.
	OVERHEATED : The spark plug face shows the insulator nose which has a white glazed or glossy appearance. Small black deposits accumulate on the insulator nose and the electrodes are prematurely worn. Lean air - fuel mixture, over advanced ignition timing or poor engine cooling efficiency are conditions that cause an overheated spark plug appearance.
	MELTED : The spark plug face shows the center and/or ground electrode surface is uneven and cauliflower-like in appearance. The insulator is blistered and possibly contains metallic deposits. The melted firing end results from overheating.
	DEPOSITS : ( Ash fouling ) A build up of combustion deposits on the firing end due to the burning of oil and or fuel additives during normal combustion. In some circumstances these deposits may reach temperatures which may lead to pre-ignition. When the deposits become heavier, ignition misfire may occur.
	OIL FOULED : The spark plug face shows wet, black, oily carbon deposits on both the insulator nose and electrodes. Oil entering the combustion chamber due to mechanical wear of piston rings, worn valve guides, or, a build up of crankcase pressure can cause this spark plug appearance.
	INSULATOR BREAKAGE : The insulator nose is cracked or split. This can be caused by ( other than mechanical impact ), thermal shock ( sudden heating or cooling ). Severe detonation will often produce this type of breakage. A lean air-fuel mixture, insufficient fuel octane rating or over advanced ignition timing can can cause this type of plug damage.
	LEAD FOULED : ( engines using leaded fuel ) Lead fouling usually appears as yellowish brown deposits on the insulator nose. May cause misfire during rapid acceleration or under heavy engine load as the lead deposits become electrically conductive at high temperatures which causes a reduction in the insulation resistance between the center and ground electrodes.
	NORMAL WEAR : The center and ground electrodes are worn. The insulator nose and ground electrode are covered with combustion deposits. The plug has operated under normal conditions for a long time and has reached the end of its life. Further usage will cause ignition misfire, poor fuel economy, and poor engine performance.
	LEAD EROSION : ( engines using leaded fuel ) The ground electrode is badly worn and its appearance is chipped. Lead additives in the fuel chemically react with the nickel alloy electrodes at high temperatures and separate the grain boundary of the nickel alloy.
	ABNORMAL EROSION : The center and/or ground electrodes are badly worn relative to the length of plug operation. Ignition misfire will occur due to the high voltage required needed to bridge the large gap. Harsh fuel and oil additives combined with severe operating conditions such as high speed driving cause abnormal erosion of the plug.
	RAPID ABNORMAL EROSION : The center and/or ground electrodes are very badly worn. The surfaces are rough and oxidised in appearance. Rapid abnormal erosion is produced by a more aggressive operating environment than " abnormal erosion ".

VACUUM GAUGE READINGS AND INTERPRETATIONS

note : The vacuum gauge readings are measured in inches of mercury at sea level.In all 4-cylinder engines, a slight fluctuation will always be noticeable.This can be overcome by pinching the gauge hose slightly to enable a steadier reading.

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	NORMAL MOTOR: steady reading of 17-21 when motor idling.		NORMAL MOTOR: when throttle is opened and closed rapidly needle falls to 2 and swings back to 24 or 25, falling back to normal idle reading.Indicates rings and valves ok
	POOR RINGS: motor idling, hand reading steadily, but 2-4 points lower than normal.This may also indicate poor or contaminated engine oil.		POOR RINGS/POOR OIL: when throttle is opened and closed rapidly needle falls to 0 and rises to only 24 or less.
	STICKING VALVE: needle drops occasionally about 4 points at idle speed.		BURNT VALVE: needle drops regularly by several points at idle speed.
	LEAKY VALVE: needle drops 2-4 points when valve should close. short circuiting individual spark plugs will indicate cylinder in which the valve is defective, when engine idling.		WORN VALVE GUIDES: rapid fluctuation of needle between 17 and 21, when engine idling.

VACUUM GAUGE READINGS AND INTERPRETATIONS

note : The vacuum gauge readings are measured in inches of mercury at sea level. In all 4-cylinder engines, a slight fluctuation will always be noticeable. This can be overcome by pinching the gauge hose slightly to enable a steadier reading.

	WEAK VALVE SPRINGS: engine running fast, the needle moves between 10 and 21.Further increase in engine speed increases needle variations.		LATE VALVE TIMING: with engine running at a higher idle speed needle reads from 8 to 15 and remains steady.
	RETARDED IGNITION: needle gives steady reading between 17 and 19 with engine running at a higher idle speed.		DEFECTIVE IGNITION: if needle moves slowly between 14 and 16 at normal idle, check ignition components (plug gaps closed up/fouled,contacts not synchronized).
1	INTAKE SYSTEM LEAK: with engine running at idle or fast idle speeds needle reads between 3 and 5. Check induction manifold gaskets.		HEAD GASKET LEAK: (leaking between cylinders): needle moves backwards and forwards regularly between 5 and 17.
	EXHAUST SYSTEM BLOCKAGE: high reading when engine first started, falling back to 1 or zero and gradually increasing to 15 or 17.		INCORRECT CARBURETTOR ADJUSTMENT: needle moves slowly backwards and forwards between 18 and 21.Check carburettor idle mixture.