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Next step after a low compression reading

Aviation Mechanic Powerplant (AMP) practice question 1 of 9 on Engine Inspection. ACS element AM.III.C.K6 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONA differential compression check with the regulator set at 80 psi produces a reading below the minimum specified for one cylinder. What should be done next?

  1. ARe-check it after running to takeoff power.
  2. BStake the valve with a fiber drift and hammer.
  3. CRemove and replace the cylinder assembly.
Show the answer and explanation

Correct answer: A. Re-check it after running to takeoff power.

  • A — correct If low compression is obtained on any cylinder, the engine is turned through with the starter or re-started, run to takeoff power, and the affected cylinder is re-checked.
  • B — incorrect Staking the valve comes later, only after the re-check still shows low compression and the valve clearance has been ruled out, and only if the manufacturer approves.
  • C — incorrect Replacement is the last resort after the cylinder has been re-checked and the cause has been investigated, not the immediate response to one low reading.

BACKGROUND

The differential pressure tester measures the leakage through a cylinder, using the principle that a given airflow through a fixed orifice produces a constant pressure drop across that orifice. The tester is connected to a 100 to 150 pounds per square inch (psi) air supply and the regulator is adjusted to 80 psi, with the piston placed at top dead center on the compression stroke. If a cylinder reads below the minimum specified, the engine is turned through with the starter or re-started and run to takeoff power, and the cylinder is re-checked. If the low reading remains, the rocker-box cover is removed and the valve clearance is checked; only then may the valve be staked with a fiber drift, and only if the manufacturer approves that procedure.

STUDY TIP

Memorize the troubleshooting order for a low cylinder: re-run and re-check, then valve clearance, then staking the valve if approved, then further investigation of the leak path.

KEY POINTS

  • The differential tester is supplied with 100 to 150 psi and regulated to 80 psi
  • A low cylinder is re-checked after running the engine to takeoff power
  • Valve clearance is checked before any valve staking, which needs manufacturer approval

SOURCES

  • amp-powerplant/ch10.txt "Differential Pressure Tester"
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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.