Engine: Combustion Chamber Pre-Ignition (or “Uncontrolled Combustion”)

NOTE: “Pre-ignition” is used here as a general term here for a case in which ignition (triggering combustion) occurs at the wrong time (literally before the next desired ignition event, even if it’s closer in time to the previous ignition event than the next one that’s coming.) What follows is a general look at our concern with uncontrolled combustion; this is not meant as a textbook level analysis (which would come with its own more strictly-controlled terms). “Uncontrolled Combustion” is certainly a fair term to use but is not typically used in our trade.

What it Does

The combustion chamber is the part of the engine where air, fuel, spark, and compression (squeeze pressure) interact to cause the combustion events that develop the engine’s power. Pre-ignition is one of the faults that can arise in how this combustion process works. Pre-ignition refers to uncontrolled ignition events that occur in the combustion chamber. For example, instead of ignition events only happening when the spark plug is fired, a combination of fuel, compression, and hot carbon debris (or other combustion friendly deposits) in the combustion chamber results in ignition events that happen at the wrong time, and in the wrong parts of the chamber. These ignition events result in combustion (think heat, flame, pressure, and gas) events that happen at the wrong time (any time other than when the spark plug fires) and wrong location (the spark plug tip is the only place where ignition events should be initiated.) Such events can result in excessive heat and stress applied to internal engine components.

Why This Matters

Pre-ignition can be a violent event within the combustion chamber. Ignition refers to the process where a spark (or other combination of heat, pressure, and flammable material) results in a combustion event. So there’s heat, pressure, gas, and physical force involved here. Our Concerns: That uncontrolled combustion events could beat up on the engine pistons, the engine valves, and the fire rings in the head gaskets. In severe cases, that valve material and head gasket material could be burned or eroded away, and holes melted in pistons. Failure of any of these components results in the need for internal engine repairs.

Possible Problems


Raspy, “rattle can” type noise under acceleration and engine load.

Internal Engine Damage

  • Burn or melt damage to engine pistons
  • Burn/erosion damage to engine valves
  • Burn/erosion damage to head gasket (specifically the fire rings)

Lack of client awareness:

Pre-ignition or abnormal combustion is not always audible, and there is no guarantee that then engine can fully control it or will “know” to turn the check engine light on to warn of a problem.

Wrong engine oil usage:

GDI engines are more susceptible to low (vehicle) speed/high load pre-ignition if oil changes are not done with oil that is rated for LSPI (“low speed pre-ignition” protection.)