What Causes the Hissing Sound of Air Leakage at Engine Idle?
1 Answers
Engine idle hissing sound due to air leakage may be caused by engine overheating, requiring inspection of the cooling system, or exhaust system blockage, vacuum tube leakage. When the engine is running, a "hissing" sound heard at the carburetor intake air filter indicates air leakage at the intake valve. A "hissing" sound heard at the exhaust pipe or muffler indicates air leakage at the exhaust valve. The reasons for valve leakage include the following: 1. Wear: The working surface of the valve and valve seat wears or corrodes, forming spots, dents, or accumulating carbon deposits. 2. Excessive Clearance: Excessive clearance between the valve stem and valve guide causes the valve stem to wobble, preventing the valve from closing tightly. Alternatively, bending or deformation of the valve stem or misalignment of the valve head can lead to incomplete closure, resulting in air leakage and producing a "hissing" or "whooshing" sound. 3. Insufficient Spring Force: Weakened or lost elasticity of the valve spring, or a broken spring, causes improper sealing between the valve and valve seat. 4. Insufficient Clearance: Too small valve lash clearance causes the valve stem to expand when heated, pushing the tappet or rocker arm to open the valve, preventing it from fully closing and resulting in air leakage noise.