ASE A1 Engine RepairCylinder Head and Valve TrainMedium
A technician is diagnosing an engine with a persistent misfire in cylinder #2. A cylinder leak-down test is performed. With cylinder #2 at Top Dead Center (TDC) on the compression stroke, 100 psi of shop air is introduced into the cylinder. The gauge on the leak-down tester indicates a 20% loss, and a distinct 'hissing' sound is heard from the exhaust pipe. What does this indicate?
- AA leaking intake valve.
- BWorn piston rings.
- CA leaking exhaust valve.
- DA blown head gasket.
Show answer & explanationAnswer & explanation
Correct answer: C. A leaking exhaust valve.
A leak-down test that shows a percentage of leakage and a distinct 'hissing' sound from the exhaust pipe strongly indicates a leaking exhaust valve. The air is escaping past the faulty exhaust valve and out through the exhaust system.
Why the other options are wrong
- A. A leaking intake valve would produce a hissing sound from the intake manifold or throttle body.
- B. Worn piston rings would produce a hissing sound from the crankcase (oil filler cap or dipstick tube).
- D. A blown head gasket would typically produce bubbles in the coolant reservoir (if a coolant passage leak) or a hissing sound from an adjacent cylinder or between the block and head.
Cylinder Leak-Down Test
A diagnostic test that measures the percentage of compression loss in a cylinder and identifies the source of the leak (intake valve, exhaust valve, piston rings, or head gasket) by listening for escaping air.
- Measures actual sealing capability of a cylinder.
- Identifies specific leak path by listening for air.
- Performed with cylinder at TDC compression stroke.
Memory trick: Hear the hiss, know the miss, leak-down's a bliss.