Car care: spark plug basics and tips

[Article copyright Ewe Paik Leong]

The two primary functions of a spark plug are to provide the spark needed to burn the fuel-air mixture and to dissipate heat from the combustion chamber.

An important concept central to the working of a spark plug is its heat range. The heat range refers to how fast heat is removed from the combustion chamber. For high performance cars or for driving in hot climates, a colder spark plug is more suitable than a hot one. A spark plug with the right heat range provides the optimum temperature to ignite the air-fuel mixture, yet remains cool enough to resist pre-ignition.

For a spark plug to work effectively and efficiently, it must be tightened
properly. A loose spark plug cannot transfer heat from the combustion chamber to the cylinder head; worse, it may allow combustion gases to leak past its threads. A spark plug that is too tight will also be unable to transfer heat effectively. Generally, spark plugs can be torqued more on an aluminium cylinder head than on one made of cast iron. A torque of 18-22 lb/ft is fine for most 14 mm spark plugs.

When the threads of spark plugs are machine cut, it is recommended to use "never-seize" compounds on the threads. This is because machine-cut threads are sharp and have burrs that can cut new threads, even on aluminium cylinder heads. When applying any “never-seize” compound, be sure not to get it smeared the firing tip.

The physical appearance of a spark plug can indicate several things about the condition of the engine. If the plug is grey or light brown and free of deposits, the engine is in good condition.

If a spark plug is coated with fuel, or carbon deposits, it is said to be "fouled" Whatever its cause, a fouled plug can still deliver sufficient voltage to the firing tip, resulting in that particular cylinder not firing proper.

"Fouling" may be dry or wet. Thick, dry deposits are symptomatic that too rich fuel is being burned; the plug is having a heat range that is too cold; the combustion chamber is suffering from 1ow compression, from poor timing or that the plug gap is too wide.

Wet deposits can signal leak problems with piston rings, with the head gasket, or with the valve trains.

Black or silver specs, melting, breakage at the firing tip are signs of possible causes such as detonation, excessive heat or pressure in that particular combustion chamber or that the plug has suffered thermal shock.

Detonation, a serious problem, refers to the pre-mature ignition of the air-fuel mixture in the combustion chamber prior to any spark from the plug.

Detonation exerts a downward force on the pistons just when they are moving upward. The collision of forces results in severe heat, breaking or melting of the firing tip of the plug, or even worse, damage to engine components such as pistons or connecting rods. The cause of detonation is the presence of a hot spot in the combustion chamber. Further investigation is needed to find the root cause of the hot spot.

Irrespective of whether your spark plugs are fouled or not, they have to be changed regularly for the simple fact they get worn out over time. Normally, old plugs are worn out at the centre and ground electrodes. To bridge the bigger gap between these two points, the ignition system has to produce a higher voltage. This results in a greater rate of misfires or incomplete combustion cycles. The negative consequences of misfiring are loss of power, increased fuel consumption and greater emissions. How long a set of plugs should be used depends on many factors such as engine compression, type of fuel used, driving conditions, and type of material the electrodes are made of.

Although spark plugs are pre-gapped from the factory, the gap may not be right for your engine, especially if you have modified the engine. If you have installed a turbo charger or have raised the compression ratio of the engine, you need to lower the gap. On the other hand, the gap needs to be widened if you have fitted a high power ignition system. It is best to seek professional advice regarding these matters. If you want to adjust the gap yourself, always start with a wider than necessary gap to avoid the risk of detonation.

Here are three tips when deciding to replace spark plugs. First, always use spark plugs of the same brand and same heat range in all cylinders. Second, even if only a spark plug is damaged and needs replacement, the entire set has to be replaced. The use of and new plugs in different cylinders results in erratic engine performance. Third, never allow your mechanic to change spark plugs when the engine is hot. A hot cylinder head has expanded and when the spark plugs are inserted, they may be seized.

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