What is the purpose of a cylinder leak-down test?

• A. To locate the source of compression loss by pressurizing the cylinder and listening for leaks
• B. To measure exhaust backpressure
• C. To assess fuel flow to the engine
• D. To verify valve timing

Answer: (D)

A cylinder leak-down test is used to determine why a cylinder isn’t sealing by introducing pressurized air into the cylinder and listening for where the air escapes. By trapping the piston at the top of the compression stroke and applying a known amount of air, you can hear or feel through the intake, exhaust, or crankcase where the pressure is leaking. If air comes out the intake, the intake valve or its seating is leaking; if it comes out the exhaust, the exhaust valve or seating is leaking; if it leaks into the crankcase, the piston rings or cylinder wall sealing are at fault. The test directly reveals the source of compression loss, which is exactly what you need to diagnose why a cylinder isn’t producing proper compression.

This isn’t about measuring exhaust backpressure, fuel delivery, or timing. Backpressure gauges and fuel-flow tests look at different systems, and valve timing is verified with timing marks or a degree wheel rather than by listening for leaks during a leak-down test.