What are the causes of valve leakage?
4 Answers
Valve leakage is caused by: 1. Damage or detachment of the valve lock pin and lock piece, resulting in incomplete valve closure; 2. Excessive clearance between the valve stem and the guide, causing the valve stem to wobble; 3. Wear or erosion of the working conical surface between the valve and its seat ring, leading to valve leakage; 4. Bending or breaking of the valve stem, or tilting of the valve head, causing the valve to lose its sealing function; 5. Weakening or breaking of the valve spring, resulting in insufficient seating force during valve closure. Valves, also known as air valves, are divided into intake valves and exhaust valves. They serve as crucial control components in the internal combustion engine's cycle of intake, compression, power, and exhaust, and are installed within the engine cylinder block.
I've seen quite a few cases of valve leakage. The main reasons are erosion or deformation after prolonged high-temperature operation, where the edges wear out and create gaps, preventing a tight seal. Another common cause is wear on the valve seat, leading to poor sealing—like our old family car, which leaked due to carbon deposits stuck between the valve and seat. Additionally, weak valve springs can cause trouble; a friend's car had unstable idle speed for this reason. Sometimes, it's an assembly issue—either the clearance is too tight or the installation angle is off. For maintenance, it's important to clean carbon deposits promptly, check the valve stem seal's integrity, and use high-quality engine oil to reduce wear. This issue can hurt engine performance and increase fuel consumption.
Valve leakage often occurs in older vehicles that have been driven for seven or eight years, with direct symptoms being reduced engine power and increased fuel consumption. Common issues include erosion and deformation of the valve head, where high-temperature exhaust gases create pits on the valve edges. Another problem is the wear and tear of the valve stem in the guide, leading to gaps over time. Additionally, valve springs can become weak and ineffective due to fatigue, resulting in insufficient closing force and inevitable leakage. In terms of maintenance, it's crucial to regularly check valve clearance—if it's too small, the valves won't close properly when the engine is hot. Mechanics typically address such issues by first measuring cylinder pressure to identify the problematic cylinder, then either grinding the valve seat with emery or replacing the entire valve assembly.
Valve leakage is mostly caused by these three reasons: the valve itself is eroded and deformed, forming a gap; the valve spring loses its elasticity over time and cannot close tightly; or the valve stem becomes loose and wobbles in the guide, causing misalignment. Additionally, excessive carbon buildup can create a gap by lodging on the valve seat, or a damaged oil seal can allow engine oil to seep into the combustion chamber, accelerating wear. This issue can lead to unstable idling and poor acceleration, and repairing it is quite labor-intensive, requiring the cylinder head to be removed and the valve seat to be reground.
