How to Identify Piston Slap?
2 Answers
Piston slap can be identified by detecting a clear and rhythmic metallic knocking sound inside the cylinder. The sound varies with temperature changes. Piston slap is most noticeable when the engine is cold, particularly at idle speed, and tends to diminish or disappear as the engine warms up. Piston slap refers to the metallic knocking phenomenon that occurs when the pressure rise rate in the cylinder is too high. It is categorized into combustion knock (or thermal knock) and mechanical knock. Combustion knock produces a sharp metallic sound near the top dead center due to combustion-related issues. If operation continues under such conditions, the peak combustion pressure in the engine will abnormally increase, raising thermal stress on components and mechanical loads. Under impact forces, moving parts will wear excessively and eventually fail.
When my old car's engine started making a ticking sound, I suspected it was piston slap. That kind of noise resembles a metallic knocking, especially noticeable during acceleration. To determine if it's this issue, it's best to listen carefully after a cold start or at low RPM. Using a handheld stethoscope or even a screwdriver pressed against the cylinder block makes it easier to pinpoint the sound source. Sometimes accompanied by symptoms like reduced power or increased fuel consumption, it can also be distinguished from other noises like valve noise—piston slap sounds deeper and more solid. Later, when I took it for repairs, I learned that excessive clearance or severe wear requires professional diagnosis to avoid further damage. Regular oil checks and piston ring replacements can prevent it. Once unusual noises are detected, don't hesitate—go straight to the shop for inspection. This experience made me pay more attention to routine maintenance and avoid delaying until problems worsen.
