A compression test shows one cylinder is too low and a leakage test indicates excessive leakage. Air is heard coming from the tailpipe. What is the most likely cause?

The indication of low compression in one cylinder, along with excessive leakage detected through a leakage test and the specific observation of air escaping from the tailpipe, suggests that an exhaust valve is not seating properly. When an exhaust valve fails to seat, it can result in a loss of compression and allow exhaust gases to escape during the compression stroke, which is the condition observed in this scenario.

This failure may be caused by various factors such as carbon buildup on the valve seat, thermal expansion issues, or wear over time. The fact that air is heard escaping from the tailpipe directly correlates with gases that should be contained within the combustion chamber instead escaping through the valve due to improper sealing.

While other options such as broken piston rings, a bad head gasket, or a bad exhaust gasket could lead to compression loss or leakage, they typically do not manifest specifically with air escaping from the tailpipe as clearly as a problem with the exhaust valve seating would. Thus, the specific symptoms in this case point directly to the exhaust valve as the most likely cause of the issue.