If air leaks from the oil fill port during a cylinder leakage test, which fault is most likely?

• A. Worn piston rings
• B. A burnt exhaust valve
• C. A burnt intake valve
• D. Cylinder liner cavitation

Answer: (A)

During a cylinder leakage test, you pressurize the cylinder with air and observe where it escapes. If air comes out of the oil fill port, the leak path from the combustion chamber to the crankcase is open. The most likely cause is worn piston rings—the rings no longer seal properly against the bore, so pressurized air bypasses the piston rings and flows into the crankcase, then out through the oil filler breather route.

Air would not escape through the oil fill port predominantly due to a burnt exhaust valve, which would vent to the exhaust system, or a burnt intake valve, which would vent to the intake manifold. Liner cavitation isn’t a primary path for air to reach the crankcase in a leakage test. So worn piston rings best explain air exiting at the oil fill port.