If the compression in a cylinder improves after adding oil, what is the likely issue?

When the compression in a cylinder improves after adding oil, it typically indicates that the issue is related to the piston rings. Worn piston rings can lead to reduced compression because they are responsible for sealing the combustion chamber. When oil is added, it temporarily fills the gaps and improves the seal, thereby enhancing compression.

This phenomenon is often observed as a diagnostic technique: when oil is introduced into the cylinder, if the compression rises significantly, it suggests that the piston rings are worn and allowing combustion gases to escape past them. This improvement is temporary, as the oil will eventually burn off or leak out, but it is a strong indication of wear in the piston rings.

Other choices, while related to engine performance, do not exhibit the same symptoms. A leaking gasket would not improve compression with oil; it would typically lower it. Blocked fuel injectors and a faulty ignition coil more directly affect fuel delivery and ignition rather than compression within the cylinder itself. Therefore, the most accurate conclusion regarding the observed increase in compression is that it is likely due to worn piston rings.