FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
A technician performs a differential compression check on a cylinder using a calibrated orifice tester and obtains a reading of 60/80. During the test, an assistant reports a steady hissing sound from the crankcase breather/oil filler with the piston at top dead center on the compression stroke. This indicates leakage past the:
- APiston rings
- BCylinder head gasket
- CIntake valve
- DExhaust valve
Show answer & explanationAnswer & explanation
Correct answer: A. Piston rings
Air escaping from the crankcase breather during a compression check indicates that compressed air is bypassing the piston rings and entering the crankcase. Leakage at the exhaust valve would be heard at the exhaust stack, and leakage at the intake valve would be heard at the carburetor or air intake.
Why the other options are wrong
- B. A head gasket leak is usually heard or seen externally between the cylinder and case, not through the breather.
- C. Intake valve leakage is heard at the induction system/carburetor inlet.
- D. Exhaust valve leakage is heard hissing from the exhaust stack/muffler, not the crankcase.
Differential Compression Test Leak Location
Listening at various engine openings during a differential compression test helps isolate the source of air leakage past a cylinder's sealing surfaces.
- Exhaust stack hiss = leaking exhaust valve
- Intake/carburetor hiss = leaking intake valve
- Crankcase breather hiss = worn/broken piston rings
Memory trick: Follow the hiss to find the miss (leak).