In a compression test, results indicate Cylinder 4 is at 130 psi while Cylinder 3 is 70 psi. What might be the likely issue?

In a compression test, significant differences in compression readings between cylinders can indicate specific issues related to the engine's internal components. When one cylinder, such as Cylinder 3 in this scenario, shows a much lower compression value than another, this suggests that there may be a problem causing loss of compression.

A low reading of 70 psi in Cylinder 3 compared to the 130 psi in Cylinder 4 can often indicate that the intake valve is not sealing properly. If the intake valve is leaking, it allows air-fuel mixture to escape into the intake manifold rather than maintaining pressure within the cylinder during the compression stroke. This leakage results in a lower compression reading.

In contrast, issues like a leaking exhaust valve or blown head gasket would typically affect both cylinders and would present different signs during the test. Piston ring wear would also tend to cause lower compression across multiple cylinders rather than being isolated to only one. Therefore, the most likely issue, given the context of the test results, is indeed a leaking intake valve in Cylinder 3.