How to do a Cylinder Leak Down Test: Steps to Diagnose Engine Issues

Diagnosing engine-related issues can prove challenging, as the engine is one of your vehicle’s most complex and intricate components. To function properly, the engine must maintain pressure within the combustion chamber. Weak pressure can cause performance issues that, if left unresolved, can lead to misfiring and eventual failure.

The following guide explores the purpose of cylinder leak down tests, including how they work and why they’re a critical diagnostic step in diagnosing internal engine issues like valve leaks, piston ring problems, and head gasket failures. Step-by-step, you’ll discover how to perform a cylinder leak down test, interpret the results, and apply that knowledge to troubleshoot common issues uncovered during these proceedings. With the help of Identifix Direct-Hit, technicians can access the appropriate repair solutions, real-world fixes, and tips necessary to address engine compression loss.

What is a Cylinder Leak Down Test?

A cylinder leak down test determines if the engine can hold pressure, which is necessary for proper and efficient combustion. The goal of completing a leak down test is to determine what percentage of compression is lost within the cylinder. It’s vital to recognize that every engine will have some percentage of leak, but only certain (low) percentages are acceptable.

When discussing cylinder leak down tests, the topic of compression tests often arises. Compression tests determine if the engine can produce pressure. Yet there is a direct correlation between low-compression and leaking cylinders that is confirmed based on the leak down test results.

Loss of compression due to leaks can cause several performance issues, from a rough idle to misfires. Worn piston rings, head gaskets, and valves are among the causes of leaks that lead to low compression. Damage to these components can also cause them to fail prematurely.

Identifying Leak Paths

The pressurized air introduced into the cylinder makes quick work of diagnosing engine leaks. Once the system is charged, all it takes is tracking down the source of the air leaks and matching those up with the corresponding diagnosis. For example, air escaping from the following leak paths points to specific worn or damaged components:

• Oil filler cap, crankcase, PCV valve, dipstick tube, or valve cover breather: Worn or broken piston rings, as well as scored or damaged cylinder walls, may be the culprits.
• Coolant: These leaks stem from a cracked cylinder head or blown head gasket.
• Exhaust: Air escaping the tailpipe indicates a broken or failed exhaust valve(s).
• Throttle body or air inlet: Air that escapes through these places signals a serious problem with the intake valve(s).

Any and all of these problems listed above can do further serious damage to your vehicle’s engine if left unchecked. For example, air escaping from the throttle body ranks among the most serious issues, as it directly affects the amount of fuel entering the combustion chamber. Engines with broken or damaged intake valves typically run rich, which unnecessarily strains the internal components.

Interpreting Cylinder Leak Down Test Results

As mentioned previously, no engine is completely sealed against leaks. Anything less than 10% is normal and indicates a healthy engine. In addition, the test results from all cylinders should be within a few percentage points of one another. 

Results exceeding 30% or varying greatly among cylinders indicate significant issues. Leaks above 30% demonstrate the engine’s inability to seal properly. Should more than one cylinder show a high percentage of leaks, the probability of a blown head gasket is high.

If the cylinder leak test results fall between 15% and 20%, you can typically address the underlying causes before they become critical. Prompt action is vital to the engine’s longevity. However, if your cylinder leak test results are suspiciously high or don’t make sense, repeat the test once more, taking care to follow the steps sufficiently to ensure a more accurate reading.

Cylinder Leak Down Test Procedures

-step-by-step instructions for conducting the test, potentially as a numbered list:

Tools Required

• Leak down tester
• Industrial-grade air compressor
• Ratchet
• Extension
• Sockets
• Breaker bar
• Spark plug socket

1. Reach Engine Operating Temperatures

Start the engine and let it run until it reaches operating temperature. While you can still perform a cylinder leak down test on a cold engine, it’s better to get an accurate reading when the piston rings have had a chance to warm up and create a tighter seal. Once the engine has reached operating temperature, shut it off and engage the parking brake, ensuring the vehicle is in gear. 

2. Remove the Ignition Coil and Spark Plug

The cylinder leak down tester uses the spark plug hole to introduce pressurized air into the system. Remove the ignition coil and spark plug of the cylinder you’re testing—no other coils or plugs. Keeping them in place will prohibit the engine from turning freely as you pressurize the cylinder.

3. Move the Piston to Top Dead Center (TDC)

The piston must be at top dead center (TDC) in its compression stroke to accurately test cylinder leaks. Confirm the piston is in the proper position by placing the breaker bar and appropriate socket on the crankshaft pulley nut. Slowly turn the engine as you cover the spark plug hole until you feel pressure.

Align the piston at TDC and check it by moving forward in the stroke. If the piston begins to dip, you’ve passed TDC. Cycle the engine once more to reach TDC, or gently turn the engine back in the right direction.

4. Commence the Leak Down Test

Before connecting the cylinder leak down test hose to the spark plug hole, remove the breaker bar from the crankshaft pulley nut. Should the engine turn over in response to the pressurized air about to fill the cylinder, the breaker bar will not spin with the crankshaft pulley nut and potentially cause bodily harm or vehicle damage.

Ensure a tight fit on the spark plug hole before bleeding the leak tester gauge until it reads zero. Then, pressurize the cylinder. Repeat these steps for all cylinders before comparing the results.

5. Record Results from Each Cylinder

As you complete the leak down test for each cylinder, keep track of the results to review once testing is complete. It’s just as important to compare the readings to one another as it is to determine individual leak percentages. Compare your findings to the cylinder locations to further diagnose any issues that may exist.

Cylinder Leak Down Test Conclusions and Recommendations

Once you’ve completed the cylinder leak down test, reviewing your results should point you in the direction of your next task. Focus your efforts on repairing or replacing components as indicated by the test results. For instance, air escaping from the throttle body during testing requires replacing the intake valves for a better seal during combustion.

Alternative Testers

Leak down testers come in various shapes and configurations, from one- or two-gauge setups to basic and digital variants. A basic single-gauge cylinder leak down tester gets the job done, but technicians often prefer more advanced versions for greater accuracy. For example, an automated leak down tester enhances accuracy while increasing productivity.

Pinpoint Engine Issues with Confidence Using Identifix

When dealing with the complex engine issues that can arise due to the results of a cylinder leak down test, it’s best to have as many resources at your disposal as possible. With Identifix Direct-Hit®, you gain access to over 3 million confirmed fixes, expanded OEM manual courage, and a virtual hotline dedicated to assisting technicians with the most complex repairs. Contact us today to learn more about our auto repair shop software solutions and services.

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