What is the difference between valve noise and piston slap noise?
2 Answers
Valve noise and piston slap noise can be distinguished as follows: 1. Piston slap noise: A dull, rhythmic "knocking" sound can be heard in the upper-middle part of the engine, which is more pronounced during cold starts and idle operation. The noise significantly diminishes or disappears once the engine reaches normal operating temperature. 2. Valve noise: A continuous, rhythmic "tapping" sound is emitted from the top of the engine during operation, most noticeable and clear at idle speed and less obvious at high speeds. Additional information on other noises: 1. Timing gear noise: The timing gear, located either at the front or rear of the engine, produces a chaotic sound with distinct metal clashing. In severe cases, the engine's timing may become disrupted, preventing startup. Disconnecting a single cylinder's fuel or ignition has no effect on this noise. 2. Timing chain noise: At low engine speeds, a "rustling" sound can be heard in the upper-front part of the engine. The noise diminishes as the speed increases and shows little variation between cold and hot engine states. Touching the timing gear cover may produce a vibrating sensation. 3. Belt noise: During engine startup or acceleration, the belt emits a "squeaking" sound. Spraying a small amount of water on the belt causes the noise to disappear instantly.
The valve noise sounds like a small hammer rapidly tapping on a metal sheet, producing a crisp 'tick-tick-tick' sound, especially noticeable during cold starts and much lighter once the engine warms up. Piston slap, on the other hand, is completely different—it's a dull 'thud-thud' noise, like hitting an empty oil drum with a wooden hammer, and gets louder the harder you press the accelerator. If you hear this muffled sound while going uphill, the engine is likely protesting. Pay attention while driving: valve noise comes from the upper part of the engine hood, changing frequency with RPM, while piston slap originates from the lower half of the engine and suddenly appears during hard acceleration. I've experienced this before—adjusting the valve clearance tighter fixed it, but never delay addressing piston slap; repairing piston or connecting rod issues usually requires at least disassembling the engine.
