Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician performs a cylinder leakage test on cylinder #4. With the piston at Top Dead Center (TDC) on the compression stroke, 100 PSI of compressed air is applied. The gauge on the leakage tester indicates 30% leakage, and a noticeable hissing sound is heard from the oil filler cap. What does this indicate?
- AA blown head gasket between cylinder #4 and the cooling system.
- BA leaking exhaust valve on cylinder #4.
- CWorn piston rings on cylinder #4.
- DA leaking intake valve on cylinder #4.
Show answer & explanationAnswer & explanation
Correct answer: C. Worn piston rings on cylinder #4.
A hissing sound from the oil filler cap (or dipstick tube) during a cylinder leakage test indicates that compressed air is escaping past the piston rings and into the crankcase. This points to worn piston rings as the cause of the 30% leakage.
Why the other options are wrong
- A. A blown head gasket to the cooling system would cause bubbles in the radiator or coolant reservoir, not a hiss from the oil filler cap.
- B. A leaking exhaust valve would cause a hissing sound from the tailpipe or exhaust manifold.
- D. A leaking intake valve would cause a hissing sound from the throttle body or intake manifold.
Cylinder Leakage Test Interpretation (Oil Filler Cap)
Diagnosing the source of cylinder leakage by listening for escaping air at various engine locations while performing a cylinder leakage test.
- Hissing from oil filler cap/dipstick indicates worn piston rings.
- Hissing from intake indicates leaking intake valve.
- Hissing from exhaust indicates leaking exhaust valve.
Memory trick: Hiss from the cap means rings are bad, but valves are quiet, never sad!