ASE A1 Engine RepairCylinder Head and Valve TrainEasy
A customer complains of a loss of engine power, rough idle, and increased oil consumption. During a cylinder leak-down test, the technician hears air escaping from the exhaust system when testing cylinder #3. Which of the following components is most likely faulty?
- AAn intake valve that is not sealing correctly.
- BAn exhaust valve that is not sealing correctly.
- CA blown head gasket between cylinders #3 and #4.
- DA worn piston ring.
Show answer & explanationAnswer & explanation
Correct answer: B. An exhaust valve that is not sealing correctly.
Air escaping from the exhaust system during a cylinder leak-down test indicates that the exhaust valve in that cylinder is not properly sealing, allowing combustion gases to escape into the exhaust manifold.
Why the other options are wrong
- A. An intake valve not sealing would cause air to escape into the intake manifold (heard through the throttle body).
- C. A blown head gasket between cylinders would typically cause air to escape into an adjacent cylinder or the cooling system (seen as bubbles in the radiator).
- D. Worn piston rings would cause air to escape into the crankcase (heard through the oil fill cap or dipstick tube).
Cylinder Leak-Down Test
A diagnostic procedure that uses compressed air to find the source of compression loss in an engine cylinder by listening for escaping air.
- Identifies specific sources of compression loss.
- Requires compressed air and a leak-down tester.
- Listening for air escaping helps pinpoint the faulty component.
Memory trick: Where the air escapes, there the problem lays.