If Cylinder 1 has 150 psi and Cylinder 3 has 50 psi, what is the most likely cause of the low compression in Cylinder 3?

The most likely cause of the low compression in Cylinder 3, with a reading of just 50 psi compared to Cylinder 1's 150 psi, is burned exhaust valve. When an exhaust valve becomes burned, it fails to create a proper seal during the compression stroke. This sealing issue allows the compressed air-fuel mixture to escape instead of maintaining the necessary pressure, resulting in significantly lower compression readings in that cylinder.

Burned exhaust valves are often a result of overheating or insufficient lubrication, which leads to the deterioration of the valve material. This problem is commonly indicated by low compression specifically in one cylinder while others remain at normal levels, as seen in this case.

In contrast, a blown head gasket typically affects multiple cylinders, leading to a loss of compression across them or potential mixing of coolant and oil rather than only affecting one specific cylinder. A leaking intake valve also tends to cause issues in the same cylinder but often results in symptoms such as backfiring or rough idling rather than solely a low compression reading. Piston ring damage might contribute to low compression as well but usually results in a more consistent loss across multiple cylinders and is less likely to be isolated to just one cylinder like the burned exhaust valve scenario.