Today, we are talking about how a spark plug works. The ignition system produces the correctly timed high voltage pulses of electricity necessary to produce sparks. The spark plugs provide the means for the electric sparks, which ignite the petrol/air mixture in an engine’s cylinder, to occur.

A spark plug consists of a metal electrode passing through the centre of a ceramic insulator. Surrounding the lower portion of the insulator is a metal casing that is screwed into the cylinder head. Welded to the bottom of the casing, so that it is earthed to the car through the cylinder head, is another electrode. It is separated from the tip of the centre electrode by a small gap.

High Tension (HT) current from the distributor flows down the centre electrode and jumps this gap in the form of a spark. For good engine performance, the spark must be big enough to ignite the fuel mixture efficiently. This means the gap must be fairly wide, but the wider it is the higher the voltage needed to produce a spark.

Spark Plugs need to be checked regularly as the electrodes will slowly be eroded and may become coated with deposits. Other factors can cause weak or erratic sparking. A crack in the insulator, or a film of water on the outer surface, or a defective plug wire, will cause leakage of electricity and a weak spark or failure to spark.

Regular servicing will ensure that your spark plugs are changed before they become eroded and coated with deposits Click To Tweet
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Corroded Spark Plug

Its time for a service if your spark plug looks like the image above

A Petrol Engine Has Spark Plugs

A Diesel Engine Has Glow Plugs

A Guide To Engine Condition


In normal engine conditions, the spark plug deposit is a rich grey, yellow or light brown with a matt finish. The deposit is powdery and can be chipped away easily.


Beads of molten metal or the electrodes welded together indicate severely excessive heat. A spark timing problem may have arisen.


A dark oily deposit, which may be covered with a layer of black grit suggests faulty piston rings or valve guides. Oil consumption may be high.

Reference: The Car Book – AA

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A New Spark Plug

Book your car in for a service to get your plugs checked – corroded plugs result in higher fuel usage

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