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Leakage heard at the engine breather outlets

Aviation Mechanic Powerplant (AMP) practice question 12 of 13 on Reciprocating Engines. ACS element AM.III.A.K8 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONDuring a differential compression check, air escaping at the engine breather outlets indicates leakage past which component?

  1. AThe intake valve
  2. BThe exhaust valve
  3. CThe piston rings
Show the answer and explanation

Correct answer: C. The piston rings

  • A — incorrect Intake valve leakage is detected by escaping air at the air intake, not at the breather.
  • B — incorrect Exhaust valve leakage is detected by listening for air leakage at the exhaust outlet.
  • C — correct Escaping air at the engine breather outlets is the stated indication of excessive leakage past the piston rings.

BACKGROUND

Cylinders showing compression below the minimum specified must be checked further to find whether the leakage is past the exhaust valve, the intake valve, or the piston. Excessive leakage at the exhaust valve is detected by listening for air leakage at the exhaust outlet, leakage at the intake valve by escaping air at the air intake, and leakage past the piston rings by escaping air at the engine breather outlets. Next to valve blow-by, the most frequent cause of compression leakage is excessive leakage past the piston, which may occur because of lack of oil. Applying engine oil into the cylinder and around the piston and rechecking compression will show whether the cylinder can continue in service.

STUDY TIP

Match each escape point to its component: exhaust outlet means exhaust valve, air intake means intake valve, breather means rings.

KEY POINTS

  • Air at the breather outlets points to leakage past the piston rings.
  • Exhaust outlet noise points to the exhaust valve; air intake noise points to the intake valve.
  • Oiling the cylinder and rechecking can confirm ring leakage caused by lack of oil.

SOURCES

  • FAA-H-8083-32B ch10 "Differential Pressure Tester"
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MORE ON RECIPROCATING ENGINES

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.