What is indicated by a compression test on a 6-cylinder engine showing Cylinder 3 at 100 psi while others are higher?

A compression test measures the pressure within the cylinders of an engine during the compression stroke. When one cylinder, such as Cylinder 3 in this case, shows significantly lower pressure—100 psi compared to higher readings in other cylinders—it indicates a potential issue that affects the engine's ability to compress the air-fuel mixture effectively.

Piston ring damage is a strong possibility in this scenario. Piston rings are responsible for sealing the combustion chamber and maintaining compression. If the rings are worn or damaged, they can create a path for combustion gases to escape into the crankcase, resulting in a lower compression reading for that specific cylinder. This would explain why only Cylinder 3 is affected while others maintain higher pressures.

In contrast, a leaking intake valve or a burned exhaust valve could lead to similar symptoms, but they typically affect the cylinder’s performance more drastically at different RPMs or conditions. Additionally, worn valve guides primarily affect the guiding of the valves rather than directly impacting compression in such a clearly measurable way as damaged piston rings would.

Thus, the indication of Cylinder 3’s low compression at 100 psi suggests that issues with the piston rings are likely the culprit, making it pivotal to investigate further for damage or wear in that area.