CYLINDER LEAK-DOWN DIAGNOSIS
Compression tells you a cylinder is leaking. Leak-down tells you exactly where the air is escaping — and that's the difference between a $200 valve job and a $2,400 teardown.
Compression test answers "is the cylinder sealing?" Leak-down test answers "where is the air going?" That single difference changes the repair scope, the part list, and the customer quote. A 30% leak-down with hissing at the exhaust = burnt exhaust valve (head off, valve job, ~$800). The same 30% leak-down with bubbles in the radiator = head gasket between cylinders (deeper teardown, ~$1,800–2,400). Same symptom on compression — wildly different repairs. Techs who skip leak-down quote teardowns they don't need OR repair the wrong thing and eat the comeback.
Position the piston at TDC of the COMPRESSION stroke — both valves closed, rings sealing, head gasket sealing. Apply regulated shop air (~100 psi) through the spark plug hole. The leak-down gauge reads what percentage of that air escapes. Healthy = 0–10%. 10–20% = wear, monitor. Over 20% = a leak you can hear or see.
Where you HEAR or SEE the air escaping tells you the failed component: hissing at the throttle body = intake valve. Hissing at the exhaust tip = exhaust valve. Bubbles in the radiator or hissing at the next cylinder = head gasket. Bubbles in the oil filler cap or hissing from the PCV = rings or worn cylinder wall.
- Leak-down tester (dual gauge) — regulated input gauge (set to 100 psi) and cylinder-side gauge that reads percentage leakage.
- Shop air supply, 100+ psi minimum — anything below 100 at the regulator gives unreliable readings. Drain the moisture trap before testing.
- TDC tool (or breaker bar on crank bolt + socket) — required to lock the piston at top dead center, compression stroke. Mechanical TDC, not the timing mark estimate.
- Mechanic's stethoscope or hose-to-ear — for isolating where air is escaping. Hand cupped at intake/exhaust works too in a pinch.
Competent tech: 30 minutes total for a thorough 4-cylinder leak-down. Where time goes — 5 min setup (regulator at 100, plugs out, throttle wedge), 5 min per cylinder × 4 (TDC position + apply air + listen at 4 locations). Red flag: if you're past 50 minutes you're either skipping TDC verification or using an unstable air supply. Slow down and re-verify position before condemning anything.
INTERACTIVE — Find the Leak
Cyl 1 is at TDC compression. Apply 100 psi shop air, then listen at each of the four positions. The leak source tells you which component has failed.
Click each position to check it. The location that's hissing tells you the failed component.
Cyl 1 has a burnt exhaust valve.
25% leakage with steady hiss at the exhaust tail pipe = combustion gas escaping past the exhaust valve. The valve face isn't sealing against the seat. Repair = head off, valve job (seat dressing + new valve, $700–900 typical).
QUICK CHECK UNLOCKED ↓
QUICK CHECK
If the piston is at TDC of the exhaust stroke (one full crank rotation off), the exhaust valve is open and you'll hear hissing at the tail pipe — but it's a false reading. Verify TDC compression by watching the timing mark AND feeling positive resistance on the breaker bar (compression pressure pushes the piston back if you're past TDC).
Below 80 psi at the regulator gives you false-low leakage readings — there's not enough pressure to push past marginal seals. Above 150 psi can rotate the engine off TDC and produce false-high readings. Always set the regulator to 100 psi BEFORE connecting to the cylinder.
All four locations get checked, every test. A burnt valve and a head gasket leak can BOTH be present on the same cylinder. If you stop listening after the first hiss you find, you'll miss the second leak and the gasket job will fail the customer back in two weeks. Always check intake, exhaust, oil fill, AND radiator.
Stuck valves and ring carbon can give a false-high leakage reading once. Best practice: re-run leak-down on the same cylinder after rotating the engine through 2-3 cycles to let valve seats and rings reseat. If the second reading is dramatically better, the first was a one-time sticking issue, not a failed component.
Cylinder Leak-Down Diagnosis
- Leak-down isolates WHERE the leak is (valve, ring, gasket); compression only tells you THAT it's leaking.
- Healthy = 0–10% leakage. 10–20% = monitor. Over 20% = locate the source by ear.
- Hissing at intake = intake valve. Hissing at exhaust = exhaust valve. Bubbles in radiator = head gasket. Hissing at oil fill = rings or cylinder wall.
- Always verify TDC of the COMPRESSION stroke before applying air — TDC of exhaust gives a false reading.
- Set regulator to 100 psi before connecting. Below 80 = false-low. Above 150 = piston rotation.
When a compression test shows a 30% drop on Cyl 3, that's the START of the diagnosis, not the answer. Pull the spark plug, rotate to TDC compression, hook up the leak-down tester, set 100 psi. Then listen — intake, exhaust, oil filler, radiator. Five minutes of careful listening tells you whether the customer is looking at a $700 valve job or a $2,000 head gasket job. The quote you give them is grounded in physical evidence, not guesswork. Customers can tell the difference and they remember which shop got it right the first time.