Question

During a cylinder leak-down test, what is the source of the leak if air is heard escaping from the oil fill port?

Accepted Answer

When air is heard escaping from the oil fill port during a cylinder leak-down test, the source of the leak is the piston rings. A leak-down test involves pressurizing a cylinder with compressed air while the piston is at top dead center with both intake and exhaust valves closed to measure how well the combustion chamber holds pressure. The piston rings are metal seals that fit into grooves around the circumference of the piston to create a pressure-tight seal between the piston and the cylinder wall. If these rings are worn, damaged, or stuck, compressed air escapes past them from the combustion chamber, flows down into the crankcase, and travels up through the engine block to the oil fill port. The crankcase is the housing that encloses the crankshaft, and the oil fill port is the opening used to add oil to this area. Therefore, air exiting the oil fill port confirms that the seal between the piston and the cylinder wall has failed, allowing air to bypass the rings and enter the crankcase.

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During a cylinder leak-down test, what is the source of the leak if air is heard escaping from the oil fill port?