* Burnt or damaged valves: This is a very common cause. A burnt valve won't seal properly, allowing the compressed mixture to escape back into the intake manifold. A damaged valve (bent or cracked) will have the same effect.
* Warped or cracked cylinder head: A warped cylinder head prevents the valves from sealing correctly against their seats. A crack in the head allows compression to escape.
* Broken or worn piston rings: Piston rings create the seal between the piston and the cylinder wall. If they're broken, worn, or stuck, compression will leak past them.
* Damaged piston: A scored or cracked piston will also allow compression to leak past it.
* Hole in the piston: This is a more severe damage that results in a significant loss of compression.
* Head gasket leak: A blown or damaged head gasket can allow compression to escape into the cooling system, oil system, or even out into the atmosphere. This is often accompanied by other symptoms like milky oil or coolant in the oil.
* Valve stem seals: Worn valve stem seals can allow oil to leak into the combustion chamber, affecting compression and resulting in smoky exhaust. While not directly a compression leak, the oil in the chamber will reduce the effectiveness of the compression stroke.
Diagnosing the problem: A compression test is the first step in identifying the cause. This test measures the pressure in each cylinder. A low reading on one cylinder points towards one of the issues listed above. Further investigation (visual inspection of the cylinder head, valves, piston, etc.) is usually needed to pinpoint the exact problem.
