Preignition
What is preignition?
Preignition occurs when the fuel-air mixture ignites in the cylinder before the spark plug's normal, timed ignition event. It is usually caused by a residual hot spot in the combustion chamber—such as a carbon deposit, cracked spark plug insulator, or other cylinder damage that glows hot enough to light the charge early. The result is lost power, high operating temperatures, and potentially severe engine damage, because expanding gases push against the piston while it is still moving up on its compression stroke. Think of it as the engine "firing too soon" because something inside is acting like a glow plug.
Also known as: pre-ignition
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