Spark Plug Failure Analysis #2 Flashover

Spark Plug Failure Analysis #2 Flashover

What is Flashover?

Flashover, also known as Carbon Tracking is the phenomenon where the spark is discharged between the terminal nut at the top of the spark plug and the metal shell (the hex section), instead of sparking at the normal centre and ground electrodes at the base of the plug.


What Causes Flashover?

Flashover occurs when the voltage across the plug itself is higher than normal  (around 25,000V to 30,000V) and can happen when the air is highly ionized (such as on a stormy day) or because of dirt and residue inside the ignition coil boot/spark covers.

If the spark gap has widened due to terminal wear, or if the compression pressure is higher than normal (such as in a turbo engine), then a higher voltage is required between the normal electrodes and flashover becomes more likely to occur.

In practice, flashover does not usually occur because the flashover voltage is blocked by fitting a plug cap that makes a good seal with the insulator.

However, if the seal part of the plug cap deteriorates (it hardens over time), or if water droplets/vapour get inside the plug cap, flashover becomes more likely, so periodic replacement of the plug caps is usually necessary.


What Does Flashover Look Like?



Insulator Design

Most spark plugs use an elongated insulator to help reduce flashover, due to the increased resistance. However many plugs also include 4 or 5 ribbed corrugations to create an even longer path (like going up and down hills) so there is an extra added resistance from the shell to the terminal nut.

Signs of flashover are black streaks on the insulator and damage to the spark plugs boot/cover and misfires. These streaks are formed as the rubber from the cap/cover seal is melted by a spark discharge and then deposited on the insulator as shown:


What Does the Spark Plug Insulator Do?

  • Insulates the high voltage that runs through the terminal post inside the plug and helps to stop shorts to the cylinder head.
  • Provides stability to the plug’s centre electrode.
  • The upper portion of the insulator extends the plug above the cylinder head, making it more easily accessible.
  • Extends the electrical path to help prevent flashover. This upper portion may be ribbed (to further prevent flashover).
  • The bottom portion of the insulator, which supports the centre electrode, protrudes into the combustion chamber. Its dimensions take part in controlling the heat range of the plug, or its ability to remove heat picked up during the combustion process.



Flashover should definitely not be ignored and causes misfires, hesitation and poor performance. We recommend cleaning out the spark plug ‘well’ from contamination by using compressed air, then replace the spark plugs, ignition coil boots/ignition coil where applicable.

Remember no copper grease should be used on the spark plug threads on installation as this can cause incorrect torque readings causing the threads to snap. 


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