Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician is performing a cylinder leakage test on cylinder #3 of an engine that has a rough idle and a misfire. With the piston at Top Dead Center (TDC) on the compression stroke, compressed air is applied. The technician hears a distinct hissing sound coming from the engine's exhaust pipe. What does this indicate?
- AA cracked cylinder head.
- BA blown head gasket between cylinders.
- CLeaking piston rings.
- DA leaking exhaust valve.
Show answer & explanationAnswer & explanation
Correct answer: D. A leaking exhaust valve.
During a cylinder leakage test, if compressed air is heard hissing from the exhaust pipe, it indicates that the exhaust valve for that cylinder is not sealing properly, allowing air to escape into the exhaust manifold.
Why the other options are wrong
- A. A cracked cylinder head could cause various leaks (coolant, adjacent cylinder), but a direct hiss from the exhaust pipe points specifically to the exhaust valve.
- B. A blown head gasket between cylinders would cause air to hiss from an adjacent spark plug hole or the coolant reservoir, not the exhaust pipe.
- C. Leaking piston rings would cause air to hiss from the oil filler cap or PCV valve, not the exhaust pipe.
Cylinder Leakage Test - Exhaust Hiss
A hiss heard from the exhaust pipe during a cylinder leakage test indicates that the exhaust valve of the tested cylinder is not sealing correctly.
- Points to a worn, bent, or carbon-fouled exhaust valve.
- Can cause misfires, rough idle, and power loss.
- Requires removal of the cylinder head for repair.
Memory trick: Listen where the air goes, that's where the problem grows.