Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician is performing a cylinder leakage test on an engine. With cylinder #1 at Top Dead Center (TDC) on the compression stroke, 100 psi of compressed air is applied. The gauge reads 60% leakage, and a hissing sound is heard from the exhaust pipe. What does this indicate?
- AA blown head gasket between cylinder #1 and another cylinder.
- BA leaking exhaust valve on cylinder #1.
- CWorn piston rings on cylinder #1.
- DA leaking intake valve on cylinder #1.
Show answer & explanationAnswer & explanation
Correct answer: B. A leaking exhaust valve on cylinder #1.
A hissing sound heard from the exhaust pipe during a cylinder leakage test indicates that compressed air is escaping past the exhaust valve, signifying a leaking or burned exhaust valve on that cylinder.
Why the other options are wrong
- A. A blown head gasket would typically cause bubbling in the radiator (if between cylinder and coolant) or a hiss between adjacent cylinders.
- C. Worn piston rings would cause a hissing sound from the oil filler cap or dipstick tube.
- D. A leaking intake valve would cause a hissing sound from the throttle body or intake manifold.
Cylinder Leakage Test Interpretation - Exhaust Hiss
Identifying the source of air leakage in a cylinder by listening for escaping air during a cylinder leakage test, with a hiss from the exhaust indicating an exhaust valve issue.
- Hiss from exhaust = leaking exhaust valve.
- Hiss from intake = leaking intake valve.
- Hiss from dipstick/oil cap = worn piston rings.
Memory trick: Air's escape, tells the leak's shape.