Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A client reports that their vehicle's engine runs roughly and has significantly reduced power. A technician performs a cylinder leakage test on cylinder #2. With the piston at Top Dead Center (TDC) on the compression stroke, 100 psi of shop air is applied. The gauge on the leakage tester shows 40% leakage, and a distinct 'hissing' sound is heard from the oil filler cap. What does this indicate?
- AWorn piston rings
- BBlown head gasket
- CLeaking intake valve
- DCracked cylinder head
Show answer & explanationAnswer & explanation
Correct answer: A. Worn piston rings
A 'hissing' sound from the oil filler cap during a cylinder leakage test indicates that air is escaping past the piston rings and into the crankcase, which points to worn piston rings.
Why the other options are wrong
- B. A blown head gasket would cause air to hiss into an adjacent cylinder (via spark plug hole) or into the cooling system (via radiator cap).
- C. A leaking intake valve would produce a hissing sound from the throttle body or intake manifold.
- D. A cracked cylinder head might cause air to enter the cooling system or an adjacent cylinder, but not typically the oil filler cap unless the crack is in a very specific, rare location.
Cylinder Leakage Test - Oil Filler Cap Hiss
During a cylinder leakage test, a distinct 'hissing' sound from the oil filler cap or dipstick tube indicates air escaping into the crankcase, signifying worn or damaged piston rings.
- Performed with piston at TDC compression.
- Air heard from oil filler cap/dipstick tube.
- Confirms worn piston rings.
Memory trick: Oil cap hisses? Piston rings in crisis!