How to Do a Leakdown Test on Your Subaru Engine: The Ultimate Guide

Performing a leakdown test on your Subaru engine is an essential part of diagnosing any potential issues you may be experiencing. By identifying any leaks in your engine’s cylinders, you can pinpoint exactly where the problem is and what is causing it. In this article, we will guide you step-by-step through the process of performing a leakdown test on your Subaru engine.

What is a Leakdown Test?

A leakdown test is a diagnostic tool used to determine the health of an engine’s cylinders. It entails pressurizing each cylinder individually with compressed air and measuring how much pressure is lost through the various parts of the engine. This test allows you to identify the presence of any leaks in the engine, such as worn valves, pistons, or cylinder walls.

Why is a Leakdown Test Important?

A leakdown test is important for two reasons – it can help diagnose any issues your engine may be experiencing and it can prevent future engine damage. By recognizing any leaks, you can address them before they escalate into more severe problems. Over time, worn-out valves or pistons can damage other engine components, so it’s essential to catch any issues early.

When to Perform a Leakdown Test?

A leakdown test should be done anytime you experience a loss in engine performance, low compression, or if your engine is misfiring. Additionally, it’s a good idea to perform a leakdown test during a routine maintenance check-up to ensure your engine is running as smoothly as possible.

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Tools Needed for a Leakdown Test

Performing a leakdown test requires a few specialized tools, including:

  • A leakdown tester – allows you to add compressed air to the engine cylinders
  • Air compressor – needed to provide compressed air
  • Spark plug wrench – to remove spark plugs from each cylinder
  • An air regulator – helps regulate the pressure of the compressed air you are providing

How to Perform a Leakdown Test on a Subaru

Step 1: Remove the spark plugs from each cylinder you want to test.

Step 2: Connect the leakdown tester to your compressed air source, ensuring that the regulator is set to 90 PSI.

Step 3: Rotate the engine so that the first cylinder you want to test is at top-dead-center (TDC). You can do this by hand-turning the engine, or if your Subaru engine has a timing belt, it’s better to use the timing marks.

Step 4: Insert the leakdown tester hose into the spark plug hole of your first cylinder.

Step 5: Turn on the compressed air and note the pressure reading on the tester. This reading should be around 90 PSI.

Step 6: Listen to where the air is escaping. If you hear air escaping through the tailpipe, it may indicate a leak in the exhaust valve. If you hear air coming from the oil filler cap, it may indicate a leak in the piston rings.

Step 7: Record the percentage of leakage and move on to the next cylinder. Repeat steps 3-6 for all cylinders.

Interpreting the Results of a Leakdown Test

Each cylinder should ideally have around 5-10% leak-down. If the percentage is higher, it indicates that something within that cylinder is not correctly sealing. The more significant the percentage, the more urgent the issue.

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What the Results can Tell You About the Health of Your Engine

A leakdown test can identify a wide range of engine problems, such as:

  • Worn out piston rings
  • Leaking intake or exhaust valves
  • Cracked or damaged cylinder walls

By identifying these issues early, you can prevent further engine damage and save yourself time and money down the road.

Common Causes of Leaks and How to Address Them

Some common causes of engine leaks include worn-out valves or pistons, incorrect valve timing, or a blown head gasket. Depending on what caused the leak, you may need to replace various parts or perform a complete engine overhaul.


Q: How long does a leakdown test take?

A: A leakdown test usually takes under 30 minutes to complete, but this can vary based on how many cylinders you are testing and if any issues are discovered.

Q: Can I perform a leakdown test myself, or should I take my vehicle to a mechanic?

A: If you have the required tools and knowledge, a leakdown test is something you can do yourself. However, if you are unsure about what you are doing or the results, it’s always a good idea to take your vehicle to an experienced mechanic.


A leakdown test is an easy and essential way to maintain your Subaru engine’s health, allowing you to diagnose and address any issues before they become more severe. Using the information provided in this guide, you can perform a leakdown test on your Subaru engine yourself, and identify any leaks that could be causing your Subaru issues. Remember, if you discover any issues, it’s always best to address them early to prevent further engine damage.

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Billy Covington

With a passion for all things automotive, Billy is our go-to expert on Subaru performance upgrades and modifications. He's been featured in several car magazines and blogs, and his extensive knowledge and expertise make him a valuable member of our team. When he's not working on cars, he enjoys playing guitar and writing music.

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