What is the most likely cause of a cylinder showing 0 psi in a compression test?

A cylinder showing 0 psi in a compression test typically indicates a significant issue that is preventing the cylinder from building any pressure. The most likely cause for this is a broken valve or a hole in the piston, as these conditions create an immediate leak that allows air to escape without allowing for any compression to develop.

When a cylinder can’t maintain any pressure, it means that whatever is happening within the combustion chamber isn't allowing for the normal compression cycle to occur. A broken valve can prevent the valve from sealing properly, leading to a situation where compression isn't built up. Similarly, a hole in the piston allows combustion gases to escape directly from the cylinder, resulting in a zero reading on the compression gauge.

In contrast, while worn piston rings can lead to low compression readings, they would typically not result in a complete lack of pressure unless the degradation is extreme. A faulty spark plug may lead to misfires or poor engine performance but wouldn’t affect the compression reading directly. An exhaust leak does not influence the compression within the cylinder either, as it occurs after the combustion process.

Thus, the presence of a broken valve or a hole in the piston are the critical failures that lead to the total absence of pressure in that cylinder.