Have you been hearing a rattling sound, or something more like a hollow knocking when driving? If so, this is known as “engine knocking.” Engine knocking is not a sound you want to hear. It is the metallic sound the engine produces when pressure is applied to the gas pedal. It is sometimes referred to as pinging, detonation, or spark knocking.

Engine knocking occurs when a separate pocket of air/fuel mixture ignites after the spark has ignited the air/fuel mixture in the combustion chamber. When the engine knocks, or pings, it releases nitrogen oxide (NOx) and unburned hydrocarbons (HCs) into the environment. These are poisonous gases that appear in a polluted sky in a yellow-brown color. They have a negative effect on respiratory problems such as emphysema and asthma.


Using fuel with an octane rating too low for your vehicle. Did you know that the octane rating of gasoline is sometimes referred to as the AKI- anti-knock index? To maintain optimum performance, manufacturers recommend certain octane ratings for their vehicles. Pinging can occur when you use an overly lean air-to-fuel mixture, or there is a lack of exhaust gas recirculation (EGR). The EGR system neutralizes engine knocking by adding a small amount of exhaust gas to the air-to fuel mixture going in to the combustion process. This limits the peak combustion chamber temperature. If you have been using fuel with an octane rating that is too low, you can purchase an octane booster to bring it up to levels that allow for proper function. If you find that you are having troubles even using the proper octane rating, try using a different brand of gasoline. To learn the proper octane rating for your specific vehicle, refer to your owner’s manual.

Early combustion. When combustion happens too early it is often referred to as a “spark knock.” Early combustion occurs from a lean air/fuel mixture, carbon build-up in the combustion chamber, and a spark plug firing too soon. Repair Pal states that if the cylinder is firing properly, the spark plug will ignite the air/fuel mixture and a flame front starts on one side of the piston and burns across the top to the other side, which creates a rapid and evenly-expanding gas that pushes down on the top of the piston. If the air/fuel mixture ignites before the spark plug fires, the two flame fronts will collide and cause the knocking noise.

Hot engine. If the engine is too hot, it can ping. This would be another early combustion scenario. It occurs when the air-to-fuel mixture “lights off” by itself. When the cooling system fails to keep the combustion chamber temperature under control, the air-to-fuel mixture will spontaneously explode. This is sometimes referred to as “pre-ignition.”

Other problems that lead to engine knocking include:

  • Carbon deposits on cylinder walls
  • Incorrect spark plugs for vehicle
  • Worn water pump bearing
  • Failed or loose timing belt tensioner (knocks when the timing belt slaps against it)
  • Defective main crankshaft bearings
  • A cracked or broken flywheel or flex-plate that attaches the engine to the transmission
  • Air conditioning compressors may knock when they ice up or fail
  • When the pistons fire an alternator with worn rotor bearings can knock

Engine knocking can be tricky to pinpoint, and needs to be thoroughly and properly inspected to find the root cause. If you are experiencing engine knocking, please feel free to contact Meridian Automotive with any questions, comments or concerns you may have. When you address issues like engine knocking quickly, and perform regular routine maintenance, your vehicle will maintain its safety and longevity.

When you need auto maintenance or repair, don’t trust just anyone, trust the experts at Meridian Automotive. Call us at (208) 297-5573 or schedule an appointment online.

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