1. Excessive Piston Slap: Piston slap occurs when there's excessive clearance between the piston and cylinder walls. This clearance allows the piston to move sideways, creating a knocking sound. The sound tends to be more pronounced when the engine is cold and can diminish or disappear as it warms up and the metal expands, reducing the clearance. Under load or WOT, the combustion pressure forces the piston against the cylinder walls, eliminating the excessive play and the knocking sound.
2. Rod/Main Bearing Issues: Worn or damaged connecting rod or main bearings can also produce knocking noises. These bearings allow the crankshaft and connecting rods to rotate smoothly. If they become worn or damaged, it can create excessive play, resulting in a knocking sound. When the engine is under load or at WOT, the increased pressure helps to reduce this play, making the knocking sound go away.
3. Worn Piston Pins: Piston pins connect the pistons to the connecting rods. Worn or loose piston pins can cause a knocking noise when the piston changes direction at the top and bottom of its stroke. The increased combustion pressure under load or WOT helps to minimize the movement and the knocking sound.
4. Incorrect Ignition Timing: Incorrect ignition timing can cause detonation or pre-ignition, producing knocking sounds. Proper timing ensures the fuel mixture ignites at the right moment in the combustion cycle. Improper timing can cause the mixture to ignite too early (pre-ignition) or too late (detonation), creating knocking. When the engine is under load or at WOT, the demand for power often helps to correct or minimize the effects of incorrect ignition timing, making the knocking sound disappear.
5. Valvetrain Issues: In rare cases, knocking sounds that disappear under load or WOT could indicate issues with the valvetrain, such as excessive clearance between the valve stems and rocker arms or the valve seats and valves. The increased valve train loads under higher engine loads tend to reduce these noises.
It's worth noting that knocking sounds in an engine are often indicative of a problem that should be addressed. While they may disappear under certain conditions, it's important to have the engine inspected by a qualified mechanic to identify the root cause of the noise and prevent potential engine damage.
