Pre-ignition

Pre-ignition is ignition of the air-fuel mixture before the spark, caused by a glowing hot spot in the combustion chamber, such as carbon deposits, an overheated spark plug electrode or a sharp edge. The piston then has to compress an already burning charge, causing extreme heat and pressure.

It differs from detonation (knock), where the remaining mixture explodes after the spark has fired. The two are linked: detonation creates hot spots that trigger pre-ignition, and vice versa.

Causes include a spark plug with too hot a heat range, lean running, overheating, heavy carbon build-up and fuel with too low an octane rating. Symptoms are pinging, power loss and running-on after switching off. Left unchecked it can hole a piston, so use the correct plug and fuel and fix cooling or fuelling faults quickly.