What are the causes of piston knocking noise in the cylinder?
3 Answers
Piston knocking noise in the cylinder is caused by: 1. Excessive wear between the piston and cylinder leading to excessive clearance; 2. Poor lubrication of the cylinder wall; 3. Incorrect piston installation direction or piston deformation; 4. Bent or twisted connecting rod; 5. Severe carbon buildup in the combustion chamber; 6. Excessive fuel injection advance angle; 7. Blocked lubrication oil passages; 8. Overly thin cylinder gasket or excessive wear on the cylinder block top surface, resulting in increased compression ratio in diesel engines; 9. Excessively fast combustion speed of the combustible mixture, causing excessively high pressure in the cylinder. The effects of piston knocking are: 1. Loud noise; 2. Piston cracking and cylinder head deformation; 3. Increased engine compression ratio, making combustion prone to knocking; 4. Damage to the cylinder head; 5. Bent or deformed connecting rod.
Speaking of piston slap noise, I've encountered quite a few cases. The most common one is excessive clearance between the piston and cylinder wall. After prolonged driving, wear causes the gap to exceed standards, making the piston rattle with a tapping sound during up-and-down movement. There's also bent connecting rods - especially common in overhauled vehicles - where improper piston movement directly scrapes the cylinder wall. Using incorrect gasoline octane causing knocking could also be the culprit, as sudden combustion chamber pressure spikes slam the piston against the cylinder. If the noise is particularly noticeable during cold starts, it's likely insufficient oil lubrication - you'll need to check the oil pump or oil viscosity. Never ignore such abnormal noises. Last time my neighbor ignored it and ended up with cylinder scoring requiring a complete engine overhaul - a huge loss.
My car had previously made a similar ticking sound like a small hammer hitting an iron bucket. After disassembling the engine, it was found that the piston ring side clearance was too large, probably due to long-term use of inferior engine oil accelerating the wear. I've also encountered cases where the connecting rod bearing wear exceeded the limit, and loose connecting rod bolts could cause similar abnormal noises. The most frustrating incident was discovering that the piston skirt was deformed, possibly caused by long-term engine overheating and overload. A reminder to owners who lower their cars: if the chassis is too low and the oil pan gets scraped on rough roads, it can lead to insufficient oil pressure, indirectly causing poor piston lubrication and resulting in knocking sounds. Such faults require professional equipment for diagnosis and are basically impossible to fix on your own.
