Other types of abnormal combustion
•Preignition is that combustion occur before the spark of the plug. The mixture “pre-ignites” from hot spots in the combustion chamber. Possible causes include a hot exhaust valve, residual hot spark plug, or carbon deposits. This is surface ignition. The preignition sound is a dull thud (เสียงทุ้มต่า) (not a “ping”). Preignition can be caused by installing the wrong spark plug, using wrong fuel and deposits in the chamber.
•Preignition can cause backfire through the intake manifold and air cleaner.