FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
A mechanic is checking the valve operating mechanism on a horizontally opposed engine equipped with hydraulic valve lifters. If the lifter plunger can be depressed more than a slight amount by hand when the engine is cold, what does this most likely indicate?
- AAn overly tight valve spring.
- BAir in the hydraulic lifter unit or a faulty check valve.
- CThe lifter is properly functioning and filled with oil.
- DExcessive valve guide wear.
Show answer & explanationAnswer & explanation
Correct answer: B. Air in the hydraulic lifter unit or a faulty check valve.
A properly functioning hydraulic valve lifter, when filled with oil, should become rigid and not be compressible by hand (or only slightly compressible) when cold. If it can be depressed significantly, it suggests air trapped within the unit or a faulty check valve allowing oil to leak out, preventing it from 'pumping up' and maintaining rigidity.
Why the other options are wrong
- A. An overly tight valve spring would increase valve train load but wouldn't cause the lifter to be compressible by hand.
- C. If properly functioning and filled, the lifter should be rigid or nearly rigid.
- D. Excessive valve guide wear primarily affects oil consumption and valve seating, not the compressibility of a hydraulic lifter.
Hydraulic Lifter Compressibility
The ability of a hydraulic valve lifter to be compressed by hand, indicating its internal oil charge and rigidity.
- Should be rigid or slightly compressible when cold and full of oil.
- Excessive compressibility indicates air or faulty check valve.
- Affects valve lash and engine performance.
Memory trick: Oil fills the gap, air makes it soft.