5 Causes of Car Engine Knocking Noise
Hearing an alarming knocking sound in your engine? Don’t ignore it! The knocking sound could indicate a serious problem that, if left unaddressed, could result in costly repairs or even engine failure.
Read on to find out what engine knock actually is, the most common causes of engine knocking, and most importantly, practical solutions to diagnose and fix the problem before serious engine damage occurs.
What is Engine Knock?
Engine knock (also commonly known as ringing, pre-ignition, or prey detonation) is the phenomenon of uncontrolled combustion in the combustion chamber. This is often caused by extreme temperatures and pressures in the combustion chamber.
When the piston compresses the air-fuel mixture, this mixture heats up. Poor fuel quality, the wrong octane rating, or aggressive ignition timing can create more pressure than the fuel can handle, causing the fuel to ignite before the spark plug fires. This often causes a ringing noise and additional vibrations. Many people describe the sound as “marbles in a can.”
Why your car makes a knocking noise
People who hear strange noises from their engines may think they’re in the clear. Here are the 5 most common causes of knocking noises in your car engine.
1) Lean Air/Fuel Mixture
A lean air/fuel mixture is one of the most common causes of engine knocking. An engine needs a precise balance of air and fuel in the combustion chamber to run smoothly.
A lean mixture can cause the fuel to ignite unevenly and spontaneously, a phenomenon known as pre-ignition. This uneven combustion can create multiple flame fronts in the cylinder that collide with each other and create a knocking or ringing noise.
Over time, a consistently lean mixture can cause significant damage to the internal components of your engine. The excessive heat and pressure generated by pre-ignition can damage the cylinder walls, pistons, and piston rings. Simply ignoring this can result in a variety of symptoms or even complete engine failure.
2) Low Octane Fuel
The octane rating of the fuel you use plays a key role in preventing engine knocking. Octane rating is a measure of the fuel’s ability to resist pre-ignition and withstand compression before ignition. Using fuel with a lower octane rating than your car requires can cause engine knocking.
Compressing the air-fuel mixture in your engine’s combustion chamber increases temperature and pressure. High-performance vehicles, especially those with high compression ratios or forced induction systems (such as turbochargers and superchargers), often require higher octane fuels (usually 91 or higher) to operate efficiently and prevent knocking. These engines are designed to compress the mixture more, which increases power but also increases the risk of pre-ignition. The recommended octane rating for your vehicle can always be found in your vehicle’s owner’s manual or inside the fuel filler flap. Using fuel with a lower octane rating than specified can result in poor performance, reduced fuel efficiency, and even damage to your engine over time.
3) Incorrect (or incorrectly adjusted) spark plugs
Every engine is designed to run with spark plugs that have a certain heat rating related to the spark plug’s ability to remove heat from the combustion chamber.
If a spark plug is too “cold” for the engine, it will not be able to dissipate heat effectively, resulting in pre-ignition and knocking. On the other hand, if the spark plug is too “hot”, it can cause detonation and engine damage over time.
In addition, using a spark plug with an incorrect electrode gap can cause engine knocking. A gap that is too large or too small can cause incomplete combustion, misfires, and knocking.
To avoid spark plug-related engine knocking, always use the type of spark plug recommended by your vehicle manufacturer. You can find this information in your owner’s manual or get it from a professional mechanic.
4) Carbon Deposits
Carbon deposits on the wall of cylinder number 4 (right) are a classic sign of a leaking head gasket. Over time, carbon deposits can build up on various engine parts, including spark plugs, valves, and pistons. These deposits are a by-product of the combustion process and can cause engine knocking if left unaddressed.
5) Lack of Cylinder Lubrication
Proper lubrication is necessary for the proper operation of the internal components of your engine, especially the cylinder head area. Without proper lubrication in the top of the cylinder head, including the valves and lifters, metal-to-metal contact and engine knocking can occur.