FAA Powerplant Written Practice Test

What does it indicate if air is heard from the crankcase breather during a differential compression check?

1. Piston ring leakage

2. Intake manifold leak

3. Valve seat leakage

4. Oil pressure failure

The correct answer is: Piston ring leakage

Hearing air from the crankcase breather during a differential compression check indicates piston ring leakage. When the engine is tested, if there is significant leakage past the piston rings, the compressed air will escape into the crankcase, leading to a distinct sound coming from the breather. This air movement signifies that the air is bypassing the rings, thus reflecting a loss of compression which is a clear indicator of worn or damaged piston rings. In contrast, the other choices pertain to different issues. An intake manifold leak would allow air to enter the system, but it wouldn’t create a sound from the crankcase breather. Valve seat leakage would generally produce noise in other areas rather than coming from the crankcase. Lastly, oil pressure failure is related to the oil system and does not directly involve air movement from the crankcase during a compression test.