FAA Powerplant Written Practice Test

If low compression is obtained on a cylinder during a compression check, what should be done?

1. Replace the spark plug

2. Run the engine and check again

3. Adjust the valve clearance

4. Perform a differential check

The correct answer is: Run the engine and check again

Low compression in a cylinder during a compression check indicates that there may be an issue affecting the cylinder's ability to properly seal and compress the air-fuel mixture. The correct step following this observation is to run the engine and check again, as this may allow the components within the engine, such as the piston rings and valves, to expand or seat more effectively under operating conditions. Engine components are designed to operate under heat and pressure, which can sometimes improve sealing and hence temporarily increase compression readings. Additionally, if the initial compression check was conducted when the engine was cold or at an improper state, running the engine could yield more accurate results. If low compression persists after running the engine, further diagnostic actions would be necessary, such as conducting a differential compression check or investigating other mechanical issues, but the initial running of the engine is crucial to ensure that the findings are accurate.