A vehicle fails a compression test with readings of 80, 75, 70, and 65 psi across cylinders. What is the most likely cause of this compression loss?

The scenario presented with compression readings of 80, 75, 70, and 65 psi across the cylinders indicates a significant loss of compression, which is generally indicative of issues related to the internal components of the engine. Worn piston rings is the most likely cause for this loss.

When piston rings wear down, they can no longer maintain a proper seal between the piston and the cylinder walls. This can lead to gases escaping from the combustion chamber during both the compression and power strokes, resulting in low compression readings. The lower the compression pressure, particularly when the readings are consistently low across multiple cylinders, suggests that worn piston rings are allowing gas blow-by, which would manifest in reduced performance and efficiency.

In contrast, valve leakage, fuel injector failure, and piston damage can also contribute to issues with engine compression, but they typically present with different symptoms or patterns. Valve leakage would usually cause uneven readings because it would affect specific cylinders depending on the condition of the valve seating. Fuel injector failure would primarily result in issues related to fuel delivery rather than direct compression loss, while piston damage could manifest with symptoms like knocking or misfiring but may not necessarily lead to uniform low readings across all cylinders as seen in this case.

Therefore, the pattern of consistent