What is likely the cause of increased compression during a wet compression test?

Increased compression during a wet compression test typically indicates that the piston rings are not sealing properly and allows more compression to be achieved with the addition of oil. When oil is added into the cylinder during the test, it provides better lubrication and can temporarily seal the spaces where the worn piston rings allow air to escape. This results in higher compression readings compared to a dry test, where only the air-fuel mixture is present.

Worn piston rings allow for compression loss because they are unable to maintain a tight seal against the cylinder walls. When performing the wet compression test, the addition of oil can restore some sealing, reflecting increased compression readings. This phenomenon highlights the condition of the piston rings and their impact on the engine's ability to maintain proper compression levels.

Other potential causes of compression issues like a burnt valve or a leaking head gasket would typically result in decreased compression rather than increased readings. Similarly, a perfectly functioning engine would show consistent compression levels without the need for added oil. Thus, the scenario described aligns directly with the characteristics of worn piston rings and how a wet compression test can reveal this issue.