A compression test tells you a cylinder is weak. A leak-down test tells you where the leak is going. It's the diagnostic that separates parts-cannon mechanics from engine builders.

How it works: With the piston at TDC on the compression stroke (both valves closed), you feed regulated shop air into the cylinder through the spark plug hole. A dual-gauge tester compares regulated inlet pressure against cylinder pressure. The difference — expressed as percentage leakage — tells you how well the cylinder seals.

Reading the escape route is the real magic. With air pressurizing the cylinder, listen and feel:

• Air hissing from the throttle body/intake: Leaking intake valve — burned seat, bent valve, or carbon deposit holding it open.
• Air from the tailpipe: Leaking exhaust valve — extremely common on high-mileage engines running lean or on gaseous fuels.
• Air bubbling in the coolant reservoir: Head gasket breach or cracked head/block into a water jacket.
• Air blowing out the oil filler or dipstick tube: Ring seal failure — worn rings, broken ring, scored cylinder wall, or a hole in the piston.
• Air from an adjacent spark plug hole: Head gasket failure between cylinders — the dreaded "combustion cross-talk."

Rule of thumb for interpretation (with the engine warm):

• 0–10% leakage: Excellent. Fresh or well-maintained engine.
• 10–20% leakage: Acceptable for daily-driver mileage.
• 20–30% leakage: Worn but functional; investigate the escape path.
• Above 30%: Something is measurably broken.

Concrete example: A shop gets a 2008 Subaru EJ25 with a P0301 misfire code and 18 psi of compression difference between cylinders 1 and 2. Compression test alone would just confirm "cylinder 1 is low." A leak-down test shows 45% leakage with air roaring out the tailpipe — burned exhaust valve, a classic EJ25 failure from lean cruise conditions overheating the seat. The tech now knows to pull the head, not throw injectors and coils at it.

Critical setup detail: The piston must be exactly at TDC. Even a few degrees off, and the piston moves down when pressurized — the engine will try to rotate. Put the transmission in gear (manual) or hold the crank with a breaker bar. Many rookies get bogus readings because the engine spun to BDC mid-test.

Do the test both cold and warm when hunting intermittent issues. A valve that seals cold but leaks hot points to a stretched valve or a seat with thermal distortion.