Detonation is a form of abnormal combustion that results from too much heat and pressure in the combustion chamber. The fuel ignites spontaneously causing a sudden rise in cylinder pressure. The result is a sharp hammer-like blow on the piston that produces a metallic knocking or pinging noise. Light detonation is considered normal and shouldn't cause any damage, but heavy or prolonged detonation can crack rings, pound out piston ring grooves, punch holes through the tops of pistons, smash rod bearings and blow head gaskets.

Detonation is sometimes confused with pre-ignition, which is altogether different. Pre-ignition occurs when a hot spot inside the combustion chamber ignites the fuel before the spark does. The hot spot may be an overheated exhaust valve, a spark plug that's too hot or even a sharp edge in the combustion chamber itself. Such hot spots can be caused by anything that makes the engine run hotter than normal or inhibits normal cooling (such as a buildup of carbon deposits). A hot exhaust valve may be the result of insufficient valve lash, a weak valve spring, excessive wear of the valve stem or guide, or retarded ignition timing. Pre-ignition can be a contributing factor in detonation.

Detonation can have numerous causes. One of the most common ones is loss of EGR. The exhaust gas recirculation system dilutes the air/fuel mixture slightly to lower combustion temperatures when the engine is under load. This reduces the formation of oxides of nitrogen (NOX) and also helps prevent detonation. So anytime you find evidence of detonation damage, be sure to check the operation of the EGR valve and system.

Other causes of detonation include excessive compression, elevated engine operating temperature, pre-ignition, over-advanced ignition timing (spark knock), lean fuel mixture, spark plugs that have too hot a heat range for the application, low octane fuel, and even bad driving habits such as lugging the engine excessively with a manual transmission.

If you find detonation damage in an engine and discover a heavy accumulation of deposits in the combustion chamber, it may be the result of a rich fuel mixture and/or oil burning. Frequent short trip driving can also accelerate deposit formation. Black, oily deposits in the combustion chambers and on the backs of the intake valves would point to worn valve guides and seals as the underlying cause. Black dry carbon deposits should lead you to check for conditions that may be causing the fuel mixture to run rich (a bad oxygen sensor, a defective coolant sensor that keeps the computer in open loop, excessive fuel pressure in a fuel injected engine, etc.)

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