How To Perform A Cylinder Leak Down Test (2024)

Understanding Cylinder Leak Down Test

The cylinder leak down test is an advanced form of the compression test, providing more detailed and accurate information about the condition of an engine. A leak down test grants a deeper understanding by revealing where compression is being lost within an engine. This information is instrumental in accurately diagnosing cylinder head problems, which could go unnoticed in a standard compression test.

While both compression and leak down tests rely on the fundamental principle of observing a pressure change in the cylinder, their execution and what they reveal differ significantly. A compression test measures the peak pressure within a cylinder at the point of ignition when the engine is turning only at a specific speed. On the other hand, a leak down test enables a mechanic to observe how much pressure is lost at a given amount of time, providing insights into leaks occurring at different stages of the engine’s operation.

Preparation and Equipment

You’ll need a quality cylinder leak down tester kit that’s appropriate for your vehicle’s engine, and a precise air compressor capable of delivering a constant and regulated air pressure. Additionally, have a breaker bar and socket for hand-cranking the engine, the vehicle’s service manual, and safety gear, particularly if you’re working with high-pressure air systems.

Before starting the test, ensure you’re working in a safe and clean area. Remove the spark plug wires or coil-on plugs to prevent accidental starts, and secure anything that could be affected by air pressure including the radiator cap, oil fill cap, and any loose parts in the engine bay. It’s also good practice to warm up the engine beforehand, allowing the piston rings to expand and seal, providing a more accurate reading.

Performing the Cylinder Leak Down Test

Here’s a detailed step-by-step guide to performing a cylinder leak down test:

• Step 1: Identify the cylinder you want to test. Rotate the engine to TDC, or top dead center, of that cylinder’s compression stroke. Leave the remaining spark plugs in place to make it easier to hand-crank and keep the engine from free-spinning too quickly.
• Step 2: Install the test equipment. Remove the spark plug from the cylinder and screw in the leak down tester. Disable the fuel system and ignition to prevent any accidental starts during testing.
• Step 3: Add air. Connect the air supply to the leak down tester and adjust the regulator to match the expected working pressure range, usually about 100 psi. Always ensure the gauge reads zero before the test begins.
• Step 4: Begin the test. With the gauge connected and the engine at TDC, you can begin the test. Observe the gauge pressure reading and the percentage of leak-down indicated on the other dial. The percentage will suggest the extent of leaks present in the cylinder.
• Step 5: Repeat on the remaining cylinders as necessary. If you’re checking more than one cylinder – as is a great idea – repeat steps 1 through 4 for the remaining cylinders.

Leak down test results are typically represented as a percentage, indicating the amount of pressure that is lost from the cylinder. Ideally, you want to see a leak percentage under 10 percent, which could be noise or a slight ring or valve seal leak. In contrast, a value over 20 percent may suggest worn components, damaged valves or seats, or other internal problems. Greater than 30 percent indicates serious issues that require immediate attention.

Troubleshooting and Interpreting Results

If your initial tests reveal high leak percentages, it’s time to trace the source of the leak. Common causes include damaged valves or seats, holes in the pistons, gouged cylinder walls, or leaky head gaskets. For head gasket tests, you can perform the test on multiple cylinders or conduct a fluid examination for oil contamination. For valve problems, a cylinder pressure balance test will help isolate the bad cylinder.

A leak down test can highlight a variety of issues based on the results:

• If multiple cylinders exhibit the same high leak percentage, it’s likely that you’re dealing with a problem that’s common to all cylinders, such as a damaged head gasket.
• A single cylinder with a high leak percentage suggests a focused problem within that cylinder, which could range from a defective valve to a cracked piston.
• Occasionally, the test might reveal exterior leaks, such as air escaping through the throttle body or exhaust. This can be due to open valves, poorly sealed intake or exhaust manifolds, or exhaust valves not in the correct position.

By understanding the complexities of the cylinder leak down test, you are better equipped to ensure the longevity and performance of your vehicle’s powerplant. Practice safety first and always use the results of the test to guide your next steps in conducting proper engine maintenance or repairs.

Need parts to repair your engine or a leak down tester? At AutoZone, we’ll have what you need on hand, and check with our Loan-A-Tool program if you’d like to borrow items like a leak down or compression tester.

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