What is the phenomenon of severe piston slap noise during cold start that disappears after a while?
3 Answers
Cold start piston slap noise that disappears shortly after belongs to engine cold start piston slap. Its characteristics are: after a cold start, rhythmic metallic knocking sounds ("clang clang") occur in the upper part of the engine at idle speed, which disappear when slightly accelerating, and the noise weakens or disappears completely as the engine temperature rises. Below are the relevant causes: 1. The main reason is poor lubrication between the piston and cylinder wall during cold start, or excessive clearance between piston and cylinder wall, or excessive piston cylindricity, causing the piston to swing and hit the cylinder wall under high-pressure gas during the power stroke, resulting in metallic knocking sounds. 2. The materials of the piston and cylinder block also affect this sound. If both are made of the same material (both aluminum alloy or both cast iron), the sound is minimal or almost nonexistent; if the piston is aluminum alloy and the cylinder block is cast iron, this sound is more common. 3. Generally, this situation rarely occurs in new cars during cold starts. If such phenomenon appears in aged engines, it indicates aging of internal engine components, where increased clearance between piston and cylinder wall leads to louder noises.
I noticed severe piston slap noise during cold starts, which disappears once the engine warms up. This is commonly referred to as 'cold piston slap'. The primary cause is the larger clearance between pistons and cylinder walls at low temperatures, where thermal contraction causes piston vibration and noise during operation. After the engine warms up and metals expand, the clearance reduces, eliminating the sound. This issue frequently occurs in older vehicles or high-mileage engines due to cylinder wear, overly thin or inappropriate oil viscosity (e.g., winter oil being too thin for adequate protection), or carbon buildup. Don't ignore it, as it may lead to piston ring damage or reduced engine lifespan. Immediate checks of oil condition are recommended, ensuring seasonal suitability and regular maintenance to prevent accelerated wear. Short-distance driving exacerbates the risk, while prolonged warm-up drives can alleviate it.
My car has always been like this. On cold winter mornings, it makes a knocking sound like a wooden fish for a few seconds when starting, but quiets down once warmed up. The issue lies in the larger cold clearance causing piston wobble noise, which disappears as the gap shrinks with heat expansion. This is mostly due to piston group wear or oil problems (oil being too thin with low viscosity and insufficient protection). Switching to higher viscosity oil significantly improved my situation. Remember to change the oil and filter promptly during each maintenance to prevent long-term wear leading to major repairs; ignoring it may cause the engine to fail prematurely and add stress while driving. Older cars especially need sufficient warm-up time, and using low-temperature oil in cold weather is advisable.
