A compression test tells you a cylinder is weak. A leak-down test tells you why — and that distinction is the difference between replacing a head gasket and replacing rings you didn't need to touch. If you've got a rough idle, an oil consumption problem, or a compression test that came back low on one or two cylinders, a leak-down test is the next step before you buy a single part.
Tool You'll Need
- Cylinder Leak-Down Tester & Compression Leakage Detector Kit — dual gauge setup that reads how much of your supplied air pressure is escaping, and gives audible/visual clues to where
You'll also need a compressed air source, the correct spark plug (or glow plug, for diesels) socket for your engine, and a way to bring each cylinder to TDC on its compression stroke — typically a socket on the crank pulley.
How This Differs From a Compression Test
A compression test cranks the engine and measures peak pressure the cylinder can build on its own — useful, but it only tells you a number is low, not why. A leak-down test does the opposite: it feeds a known, regulated amount of compressed air into the cylinder from outside and measures how much of it escapes. Where it escapes (and how) tells you exactly what's failing.
Step 1: Remove the Spark Plugs
Pull all the spark plugs (or glow plugs) so the engine turns freely by hand — you'll need to rotate it precisely for this test, and compression from other cylinders will fight you if the plugs are still in.
Step 2: Bring the Target Cylinder to TDC
Rotate the crankshaft by hand until the cylinder you're testing is at Top Dead Center on its compression stroke — both valves closed. This matters: testing a cylinder that isn't at TDC gives you a false reading since the valves may be partially open.
Step 3: Connect the Tester and Set Zero
Thread the tester's fitting into the spark plug hole, connect your air supply, and follow the tool's calibration step to zero the gauge before applying full test pressure — this accounts for any restriction in your specific air line and fitting.
Step 4: Apply Air and Read the Gauge
Apply regulated air pressure and read the percentage of leakage on the gauge. As a general rule of thumb, under roughly 10% leakage is considered healthy on most engines, 10–20% is worth monitoring, and above 20–25% points to a real problem — but treat these as general guidance rather than a hard pass/fail line, since acceptable leakage varies somewhat by engine design and mileage.
Step 5: Listen and Look for Where It's Escaping
This is the actual diagnostic power of the test:
- Hissing from the intake — points to a leaking intake valve
- Hissing from the exhaust — points to a leaking exhaust valve
- Bubbles in the coolant expansion tank — points to a head gasket leak between the cylinder and a coolant passage
- Air coming out the oil fill cap or dipstick tube — points to worn piston rings or a scored cylinder wall
- Air from an adjacent spark plug hole — points to a head gasket leak between two cylinders
Step 6: Repeat for Every Cylinder
Test all cylinders and record the percentage and the escape point for each, even the ones you don't suspect — a comparison across cylinders is often more useful than any single number in isolation.
Step 7: Reassemble
Reinstall the spark plugs to the correct torque and gap, reconnect what you disturbed, and use your results to decide the actual repair — not a guess.
Common Mistakes
- Testing a cylinder not at TDC — gives a meaningless reading with valves partially open
- Skipping the zero/calibration step — your percentage reading is only as good as your baseline
- Only testing the cylinder you already suspect — comparing across all cylinders reveals patterns a single test misses
- Ignoring where the air is escaping — the percentage tells you how bad it is, the location tells you what it is
Browse our full specialty diagnostic tools lineup for more engine troubleshooting essentials.