A compression test shows that one cylinder is low. When performing a cylinder leakage test, air is heard coming out of the dipstick tube. This would indicate

A compression test shows that one cylinder is low. When performing a cylinder leakage test, air is heard coming out of the dipstick tube. This would indicate

• A. Worn exhaust valve
• B. Broken piston rings
• C. Leaking head gasket
• D. Cracked cylinder head

Answer: (B)

During a cylinder leakage test, you pressurize a cylinder and listen for where the air escapes to identify the leakage path: past the valves, past the piston rings into the crankcase, or into the cooling system via a head gasket. If air comes out of the dipstick tube, it means the air is entering the crankcase. The only usual path for air to reach the crankcase under compression is past the piston rings, so the rings aren’t sealing properly. That points to worn or broken piston rings. Air leaking through the exhaust valve would show up at the exhaust system, not the dipstick. A leaking head gasket would typically route air to the cooling system or into an adjacent cylinder, not into the crankcase. A cracked cylinder head would cause other symptoms like coolant loss or crossflow between chambers, not a dipstick vent.

During a cylinder leakage test, you pressurize a cylinder and listen for where the air escapes to identify the leakage path: past the valves, past the piston rings into the crankcase, or into the cooling system via a head gasket. If air comes out of the dipstick tube, it means the air is entering the crankcase. The only usual path for air to reach the crankcase under compression is past the piston rings, so the rings aren’t sealing properly. That points to worn or broken piston rings.

Air leaking through the exhaust valve would show up at the exhaust system, not the dipstick. A leaking head gasket would typically route air to the cooling system or into an adjacent cylinder, not into the crankcase. A cracked cylinder head would cause other symptoms like coolant loss or crossflow between chambers, not a dipstick vent.