FAA Powerplant Written Practice Test

What does air coming from the crankcase breather during a differential compression check indicate?

1. Oil contamination.

2. Piston ring leakage.

3. Cylinder head failure.

4. Faulty oil pump.

The correct answer is: Piston ring leakage.

When performing a differential compression check, the presence of air escaping from the crankcase breather generally indicates that there is leakage occurring somewhere in the engine's combustion chamber, specifically through the piston rings. This leakage suggests that the piston rings are not sealing properly against the cylinder wall, leading to a loss of compression. In a properly functioning engine, the air should remain within the combustion chamber during the compression check, and any significant amount of escaping air through the crankcase breather points towards an issue with the piston rings' integrity. This condition can also lead to reduced engine performance and increased oil consumption due to the failure of the rings to maintain a proper seal. The other options may refer to various issues in an engine but do not directly correlate with the phenomenon of air escaping through the crankcase breather during a differential compression check, making the identification of piston ring leakage as the correct response essential for understanding engine diagnostics.