AIM Powerplant Systems I Block 14 Practice Test

Using a spark plug that runs too hot keeps the plug tip at a higher temperature than ideal. That excess heat can cause the fuel–air mixture to ignite before the spark plug fires, a phenomenon known as preignition. Preignition creates higher peak pressures and temperatures inside the cylinder, which can lead to knocking and potential engine damage if it occurs repeatedly or severely. Fouling happens when a plug is too cold and can’t burn off deposits, so it’s not the result of a plug running hot. Issues like a burned capacitor or wiring fault are ignition-system problems separate from the plug’s heat range and aren’t caused by a plug that’s too hot.