ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A technician is performing a cylinder leak-down test. After pressurizing Cylinder 1 to TDC (Top Dead Center) on the compression stroke, a significant hissing sound is heard from the exhaust pipe. What does this indicate?
- ALeaking exhaust valve
- BLeaking intake valve
- CLeaking piston rings
- DLeaking head gasket
Show answer & explanationAnswer & explanation
Correct answer: A. Leaking exhaust valve
A leak-down test pressurizes the cylinder with compressed air. If air is heard hissing from the exhaust pipe, it indicates that the exhaust valve is not sealing correctly, allowing air to escape past it and into the exhaust manifold.
Why the other options are wrong
- B. Leaking intake valve would cause air to hiss from the throttle body or intake manifold.
- C. Leaking piston rings would cause air to hiss from the oil filler cap or PCV valve, into the crankcase.
- D. A leaking head gasket would cause bubbles in the coolant reservoir (if leaking to cooling system) or air to hiss from an adjacent spark plug hole (if leaking to another cylinder).
Cylinder Leak-Down Test
A test that measures the percentage of compressed air that leaks out of a cylinder over a specific time, and helps pinpoint the source of the leak.
- Requires compressed air and a specialized gauge set.
- Identifies leaks in piston rings, valves, or head gaskets.
- Hissing sounds from specific locations (exhaust, intake, oil fill) pinpoint the faulty component.
Memory trick: Leak-down's hiss, tells where the seal went amiss.