What are the causes of piston knocking noise?

[SerenityLynn]

The causes of piston knocking noise are as follows: 1. It may be due to excessive piston clearance or piston deformation. Since the piston clearance is larger in cold conditions, the knocking noise is naturally more severe. When the engine temperature rises, the piston expands, reducing the clearance, and thus the knocking noise diminishes. 2. It could also be caused by severe carbon buildup in the combustion chamber. Excessive carbon deposits in the combustion chamber increase the compression ratio of the diesel engine, leading to excessively high gas pressure and temperature. When atomized diesel fuel is injected into the cylinder and encounters this high-temperature, high-pressure gas, it ignites and burns, causing what is known as detonation. During detonation, the flame spreads outward at an extremely high rate, even before the gas has time to expand, causing a sharp rise in temperature and pressure, forming pressure waves that propagate forward at the speed of sound. When these pressure waves strike the cylinder wall, they produce a sharp knocking noise.

[DelAva]

The sound of piston slap in the car is like metal knocking, and I've encountered it several times, most noticeable during cold starts. The main reason is the increased clearance between the piston and cylinder, possibly due to worn piston rings or insufficient expansion caused by engine overheating. If there's insufficient oil or poor lubrication, increased friction can cause the piston to swing and hit the cylinder wall. Improper assembly or prolonged use leading to part aging can also cause problems. I recommend getting it checked immediately, don't wait until the sound gets worse. Take it to a repair shop to measure cylinder pressure and clearance size with a diagnostic tool, and replace piston rings or adjust assembly if necessary. For regular maintenance, make sure to change the oil on time with good quality oil, keep the lubrication system functioning properly, and avoid prolonged high-speed driving to prevent overheating and increased wear. Ignoring this issue can damage the engine, leading to high repair costs and potential hazards.

[VonGiselle]

I encountered a piston knocking issue, which generally has three root causes. First, problems with the piston itself, such as worn or deformed rings, leading to increased clearance and unstable movement, which causes noise. Second, external factors, like uneven expansion due to engine overheating or misalignment during assembly. Third, inadequate lubrication, such as poor-quality or insufficient engine oil, which increases friction between the piston and cylinder, leading to knocking. This sound is more noticeable at low speeds or during acceleration, resembling a drumbeat. If you notice unusual noises, don’t panic—first check the oil level and temperature, and listen to see if the sound is coming from near the center of the engine. During repairs, the engine must be disassembled to inspect for wear. Minor issues can be fixed by adjusting the clearance, while major problems may require part replacement and maintenance. For prevention, using high-quality oil and regular maintenance can save a lot of trouble.

[LillianFitz]

Piston slap noise is usually caused by excessive clearance between the piston and cylinder. Possible reasons include worn or loosened piston rings after prolonged use, or material deformation due to engine overheating. Other factors such as insufficient oil leading to lubrication failure, or loose piston pins can also cause abnormal piston movement and impact against the cylinder wall. This sound is clearly audible during cold starts or under load. Timely diagnosis is essential to prevent more severe damage like cylinder wall scratches or even engine seizure. Prevention relies on proper maintenance to ensure thermal stability of the engine.