How To Perform A Cylinder Leakdown Test

Mastering Cylinder Leakdown Testing: Pinpointing Compression Loss Like a Pro

If you’ve ever run a compression test and found one or more cylinders reading lower than the rest, you know there’s a problem. But a compression test can’t tell you why the cylinder is losing pressure. That’s why I always follow up with a cylinder leakdown test to pinpoint the cause of the low compression.  This test allows me to go beyond the numbers and diagnose the root cause—whether it’s worn piston rings, a burned or sticking valve, or a failing head gasket.

A cylinder leak down test helps me assess internal engine health, isolate failing cylinders, and determine whether an engine needs a simple fix or a full rebuild. It’s especially helpful in cases where an engine is overheating, and I can’t pinpoint the root cause.

A Case Where A Blown Head Gasket Causes Radiator Damage

Here’s an example of why you need to conduct a cylinder leakdown test when you’ve got an overheating problem:

Your engine is overheating and you find a cracked/leaking radiator. That sounds like a no-brainer; you replace the radiator. But what if the radiator cracked because of a failed head gasket where the engine is pumping hot high-pressure exhaust into the cooling system? In that case, if I replace the radiator without doing any more testing, the new radiator could crack again. The customer will be back, complaining of another overheating condition and questioning my diagnostic skills.

Whether I’m diagnosing a single misfiring cylinder or evaluating an engine before a major repair, this test is an absolute necessity.

Choosing the Right Cylinder Leakdown Tester

Not all leakdown testers are the same, and I always recommend using a twin-gauge setup—one gauge measures shop air pressure, while the other shows the percentage of leakage. Some gauges may read slightly differently, but the key is keeping the shop air pressure steady throughout the test. That consistency is what really matters when diagnosing an engine with a cylinder leakdown test.

Step By Step Guide To Performing a Cylinder Leakdown Test

I have a precise routine for running a cylinder leakdown test, ensuring I get accurate and reliable results every time. Here’s how I do it:

1. Prepare the Engine— Start by warming up the engine. This ensures the piston rings expand and seal properly, providing an accurate reading. Then remove the spark plug from the suspected cylinder, but leave the others installed. This helps provide resistance against engine rotation when air pressure is applied.

2. Position the Piston at Top Dead Center (TDC)— Using a breaker bar and socket, rotate the crankshaft until the piston in the test cylinder is at TDC on the compression stroke. This is a crucial step—if the piston isn’t positioned correctly, applying air pressure will force the engine to rotate unexpectedly.

Safety Tip: Before applying air, I always remove the breaker bar and socket from the crankshaft. If the engine isn’t at perfect TDC, it can spin suddenly, causing injury or damage.

3. Connect the Tester and Apply Air Pressure— Install the leakdown tester’s adapter into the spark plug hole and connect it to a steady shop air source. Before beginning, I verify that my gauge is properly zeroed out. Ensure that shop air pressure remains between 60-100 psi—a consistent pressure is key for accurate readings.

4. Record the Leakage Percentage— With air pressure applied, I take note of the leakdown percentage for the cylinder. If multiple cylinders have low compression, I repeat the process for each one.

Interpreting the Leakdown Test Results

No engine is perfectly sealed, so I don’t expect a 0% cylinder leakdown test reading. However, the numbers tell me everything I need to know about an engine’s condition:

• 5-10% leakage: The engine is in excellent shape—no major concerns.
• 15-20% leakage: Not necessarily a problem, but I will continue to investigate where the air is escaping.
• 30% or more leakage: This tells me I have a serious issue that can be caused by a head gasket, burned valve, or the engine may need a complete overhaul.

If I’m testing multiple cylinders, I also look for consistency across readings. A difference of 4-5% between cylinders is normal, but anything greater indicates a potential issue.

How To Find the Source of the Leak

The real advantage of the cylinder leakdown test is that it tells me where compression is escaping. By listening for air leaks at key locations, I can quickly identify the problem:

Listen at the oil fill cap or dipstick tube— Hissing from under the valve cover through the oil filler cap opening or the dipstick tube signifies that the piston rings are worn. The hissing indicates that air in the cylinder is escaping past the rings, into the crankcase and then up through the oil drainage ports or dipstick tube.

Listen at the throttle body— If I hear air hissing through the throttle body, that indicates an intake valve issue. Remember, the engine is set at TDC, meaning both the intake and exhaust valves are closed. The only way I could hear hissing in the intake manifold/throttle body was if I had a bad/carboned intake valve, a broken/weak intake valve spring, or a worn cam lobe for that cylinder.

Listen at the tailpipe— If I hear air hissing from the tailpipe, the exhaust valve isn’t sealing properly. That can be caused by a worn/burned valve, carbon buildup on the valve, a broken/weak exhaust valve spring, or a worn cam lobe for that valve.

Listen in the radiator or coolant reservoir— If I hear hissing in the radiator or cooling system reservoir, and I don’t hear hissing at the tailpipe or throttle body, that’s the sign of a head gasket breach between the cylinder and a cooling port. In other words, the intake and exhaust valves are closed (verified by no hissing at the tailpipe or throttle body), so the only way air can make its way from the cylinder to the radiator or reservoir is through a head gasket breach.

Making Sense of the Results

If I find 30%+ leakdown and most of the air is escaping past the rings, the engine is likely worn out and needs a rebuild. However, if leakage is under 20% and concentrated around the rings, a rebuild may not be worth the cost—since power loss at that level is usually minor.

Intake valve leaks are a bigger concern. If I detect air escaping through an intake valve, I sometimes tap it lightly with a plastic mallet while the test is running. If that reduces the leakage, the valve may just be sticking. If the leak persists and exceeds 20%, it’s time to pull the head and make repairs.

Final Thoughts on Cylinder Leakdown Testing

When it comes to diagnosing internal engine issues, the cylinder leakdown test is one of the most powerful tools in my arsenal. It eliminates the guesswork by telling me exactly where compression is being lost and whether an engine needs repair or replacement.

That said, I don’t get alarmed by 15-18% leakdown across all cylinders—if the readings are consistent, the engine is usually fine. But if one cylinder has a significantly worse result than the others, I know it’s time to take a closer look.

At the end of the day, the cylinder leakdown test gives me the confidence to make the right call—whether it’s a simple fix or a full engine rebuild. If you’re serious about engine diagnostics, this is a test you need in your toolkit.

ChatGPT can make mistakes. Check im