What Causes an Engine Knock?

An engine knock results from one of several problems such as a loose connecting-rod bearing, bad fuel mixture, worn spark plugs or carbon build up inside the cylinder walls. A vehicle experiencing a knock should be quickly inspected to avoid any serious damage to the internal components of the engine.

Engine knock sounds vary depending on the cause, as it can be either a deep thumping noise or a light rattle. The root of the engine knock can generally be solved by locating the source of the sound. There are a number of root causes to many common engine problems.

  • Early detonation - when a spark plug fires too early, the engine will knock. This occurs when the timing of the vehicle is off or the spark plugs are worn
  • Engine overheating - when the engine's cylinders are too hot, the combustion chamber will ignite itself. This is called pre-ignition
  • Using the wrong fuel - every vehicle has a recommended octane rating; when an overly lean gas type is used, the engine will knock
  • Connecting rod failure - the connecting rod on an engine slides onto the crankshaft using a rod bearing. These bearing need to be kept properly lubricated. When one or more of the bearings fail due to oil starvation, the connecting rod will hit metal every time the crankshaft rotates. In order to be repaired, the entirety of the bottom end will need to be rebuilt