Cylinder Leakage Testing: Finding Where the Seal Is Lost

The short version — Leak-down testing pumps regulated shop air into a cylinder at TDC compression to measure percentage loss, and where the air escapes from tells you exactly which sealing surface has failed — rings, valves, or head gasket.

What the Test Actually Does

A cranking compression test tells you a cylinder is weak. A leak-down test tells you why. Cylinder leakage testing injects regulated compressed air into the cylinder at TDC compression stroke and quantifies the sealing loss as a percentage, while also letting you pinpoint the leaking surface by listening and looking for where the air comes out. This is the key reason it's tested separately from a compression test: it's a static test done with the engine stopped, not a dynamic cranking test. You're not measuring how much pressure the engine can build — you're measuring how much air a sealed, motionless chamber can hold.

The tool itself is simple: two gauges — a regulator/source gauge and a cylinder gauge — connected through a calibrated orifice, with a hose that threads into the spark plug hole (or injector hole, depending on engine). Air flows from the shop line, through the regulator, through the orifice, into the cylinder. The orifice is what makes the percentage reading meaningful — it's a known restriction, so any pressure drop on the cylinder gauge represents air actually escaping the chamber, not just line loss.

Setup: TDC and Holding the Crank

Before you inject any air, two things have to be true, and both exist for the same underlying reason — you need a fully sealed, immobile chamber.

• The cylinder under test must be at TDC on the compression stroke, so intake and exhaust valves are both fully closed. If either valve is even slightly off its seat because you're not at the right TDC, you'll get a false leakage reading that has nothing to do with wear — it's just a valve mid-cycle.
• The crankshaft must be held from turning — transmission in gear with the parking brake set, or a proper holding tool. This isn't optional bench advice; injected air pressure pushing on the piston can actually spin the crank if it's free to rotate, which changes your TDC position mid-test and can throw tools or your hand into a rotating pulley.

Because getting to TDC means hand-cranking or bumping the engine over, the ignition system should be disabled and the fuel system depressurized/disabled first, same as any procedure where you're rotating the engine by hand. This is standard safety practice, not something unique to leak-down testing, but it applies every time you set up this test.

Reading Where the Air Escapes

This is the diagnostic heart of the test — the percentage tells you how much is leaking, but the escape point tells you what is leaking.

• Air rushing out the throttle body/intake points to a leaking intake valve — it's not sealing, so air is blowing backward up the intake tract.
• Air rushing out the tailpipe points to a leaking exhaust valve, air escaping down the exhaust path the same way exhaust gas normally would.
• Air bubbling up in the coolant (radiator neck or overflow) means the head gasket has failed between the cylinder and a coolant passage, or the head/block itself is cracked — the combustion chamber and cooling system are no longer separate.
• Air hissing out the oil fill cap or dipstick tube means the rings are worn or a piston/ring land is cracked, letting cylinder pressure blow past the rings into the crankcase.
• Air hissing out of an adjacent cylinder's spark plug hole means the head gasket has blown between those two cylinders — the gasket's seal between the two combustion chambers has failed, so pressurizing one leaks straight into the other.

Each of these clues corresponds to a different repair path: valve work, head gasket/head or block replacement, or ring/piston work. Common root causes behind excessive leakage include worn or broken piston rings, valves that are burnt or not fully seated, worn valve seats, a damaged head gasket, or a cracked head or block. The leakage percentage confirms there's a problem; the escape point tells you which of these it is.

Finishing Up Safely

When the test is done, don't just yank the hose off the cylinder. Relieve any residual pressure still in the cylinder and disconnect the air supply first. Keep hands and tools away from the crankshaft/pulley area throughout the whole process — the same air pressure that can spin an unheld crank can also cause unexpected movement even with the crank nominally held, so treat that area as live until the air is fully disconnected.

Easy to mix up

• Compression test vs. leak-down test: cranking compression test is dynamic (engine turning over); leak-down/leakage test is static (engine stopped) and measures a percentage through a calibrated orifice. Don't confuse "low compression" diagnosis with "leak-down escape point" diagnosis — they answer different questions.
• Throttle body vs. tailpipe: intake valve leak exits the throttle body/intake; exhaust valve leak exits the tailpipe. Easy to swap these under test pressure — remember the air follows the same path the gas normally would (intake leak goes backward up intake, exhaust leak goes out the exhaust).
• Coolant leak vs. adjacent cylinder leak: both point to a head gasket problem, but coolant bubbling means the gasket failed into a coolant passage, while hissing at the neighboring spark plug hole means the gasket failed between two cylinders. Same root cause category, different specific failure point.

Check yourself

Question: You're leak-down testing cylinder #3 and hear a steady hiss coming from the spark plug hole of cylinder #4 right next to it. What does this tell you?

It indicates a blown head gasket between the two adjacent cylinders — the gasket's seal separating cylinder #3 and cylinder #4 has failed, letting injected air pass from one combustion chamber into the other.

Question: Technician A says the leak-down test should be performed while cranking the engine, similar to a compression test. Technician B says the leak-down test is performed with the engine stopped, at TDC on the compression stroke, with the crank held from turning. Who is right?

Technician B is right. Leak-down testing is a static test done with the engine stopped — the cylinder is set at TDC compression so both valves are closed, and the crank must be held (in gear with parking brake set, or with a holding tool) because injected air pressure can spin an unheld crank. A cranking compression test, not leak-down, is the dynamic test.

Question: During a leak-down test, air is heard escaping from the dipstick tube. What does this point to, and what would you check?

Air escaping from the oil fill or dipstick tube points to worn piston rings or a cracked piston/ring land — cylinder pressure is blowing past the rings into the crankcase. You'd want to inspect the rings and piston for wear or cracking as part of the repair.