What Are the Main Causes of Low Cylinder Pressure?

[StGabriela]

The causes of low cylinder pressure include air leakage between the cylinder head and the cylinder, leakage between the spark plug gasket and the cylinder head or a faulty decompression valve, crankcase leakage, and leakage due to severe wear between the piston rings and the cylinder or stuck piston rings. The diagnostic procedure can be carried out in the following order: 1. Air leakage between the cylinder head and the cylinder: Rotate the engine by foot and carefully observe the joint surface between the cylinder head and the cylinder for oil foam. If oil foam is present, it indicates leakage at the cylinder head gasket. In this case, check whether the tightening nuts on the cylinder head are properly tightened. If they are loose, tighten them evenly. If oil foam still appears after tightening, remove the cylinder head and inspect the cylinder head gasket for damage. If damaged, replace it with a new one. When removing the cylinder head and finding severe black carbon deposits at the leakage site, check the cylinder head for damage or warping and perform repairs or grinding. When replacing the cylinder head gasket, use original manufacturer parts. If making a gasket yourself, pay attention to the material and thickness. If made from soft aluminum plate, the material may be too hard to ensure sealing; if made from non-metallic materials, it may be easily burned or damaged by high-temperature, high-pressure gases in the combustion chamber. The thickness of the cylinder head gasket directly affects the engine's compression ratio—increased thickness reduces the compression ratio, while decreased thickness increases it. Therefore, the thickness of the cylinder head gasket must not be altered arbitrarily. 2. Air leakage between the spark plug gasket and the cylinder head or a faulty decompression valve: Damage to the spark plug gasket, surface contamination, or even forgetting to install the spark plug gasket can cause air leakage. During diagnosis, rotate the engine and carefully check around the spark plug for oil foam or hissing sounds indicating outward air leakage. If leakage is detected, clean the spark plug gasket, reinstall it, or replace it with a new one.