Leak-Down Test: What It Is, and How to Do One

To run properly, an engine needs fuel, spark, compression and exhaust. To create compression — the pressure created as the piston compresses the air-fuel mixture — the engine’s cylinders must be completely sealed. 

That means the valves must close properly and the piston rings and cylinder walls must form a tight seal. Leaks will ruin your engine’s performance.

A leak-down test is one way a mechanic can find whether an engine has an internal problem instead of something external, like a bad spark plug. The leak-down test measures pressure loss inside a cylinder. This procedure is different — and usually more accurate — than a compression test, which just measures the pressure inside a cylinder.

How to perform a leak-down test

This procedure is usually best left to a professional. But, if you have a lot of do-it-yourself experience working on engines, you may be able to use the following steps to run the test yourself.

1. Gather tools

You’ll need a leak-down tester, industrial-grade air compressor (one large enough to run air tools), a ratchet, extension, sockets, breaker bar and spark plug socket.

2. Prepare the vehicle

Make sure the engine is off. If you’re performing a leak-down test on a single cylinder, you only need to remove that particular spark plug. But if you want to do a leak down test on the entire engine, go ahead and remove all the spark plugs now. 

To remove a plug, first, take off the attached spark plug wire or coil pack. Then slide the spark plug socket, with the extension attached, over the spark plug. Attach the ratchet, and turn it counter-clockwise to loosen and remove the plug.

3. Locate top dead center

To get accurate readings from a leak-down test, the cylinder being inspected must start at top dead center. This way, both the intake and exhaust valves are closed, sealing the cylinder. 

There are a few different ways to perform this task, but if you have someone to help you, this is generally the easiest:

• Have your helper put a breaker bar and socket on the crankshaft pulley nut. This is typically the largest pulley, located at the bottom of the engine.
• Put your finger over the spark plug hole of the cylinder to be tested.
• Have your assistant turn the engine over by rotating the crankshaft pulley (usually clockwise) with the breaker bar.
• When you start to feel pressure on your finger, that means you’re on the compression stroke. At this point, stop and use a flashlight to look at the piston inside the cylinder.
• If you can’t get a good view of the piston, carefully insert a long object, such as an extension, into the cylinder. Make sure it’s sitting on top of the piston and isn’t cockeyed. 
• Instruct your assistant to turn the crankshaft pulley again. Watch the piston (or the extension) as it moves upward in the cylinder. When it comes to the top of its travel, before it starts going back down, you’ve reached top dead center. 
• If you wait a little too long and the piston begins its descent, you’ll need to turn the engine slightly in the opposite direction to get the piston back to the very top.

4. Hook up the tester  

The leak-down tester comes with a hose and a variety of adapters that screw into the spark plug hole.

• Select the correct adapter: Hold the adapter threads up to the spark plug threads to ensure they’re a match. Also, compare the diameter of the plug to that of the adapter. Then, thread the adapter into the leak-down tester hose.
• Insert the adapter: Place the adapter with hose attached into the spark plug hole. Thread it in by hand.
• Apply air to the tester: Connect compressed air to the leak-down tester. Pull the knob out of the base of the tester and turn it until the gauge that reads “cylinder leakage” is set to zero.
• Connect the tester: Pull back on the collar of the leak-down tester, and push the adapter hose into it. Once the adapter is in place, you can lock it down by releasing the tester collar.

5. Read and interpret the test results

This is where you actually get to see the test results and interpret their meaning. Look at the gauge labeled “cylinder leakage.” Generally, if it reads 20% or more, the cylinder isn’t sealing properly, meaning there’s a problem.

Repeat the above steps on each cylinder you want to test.

6. Determine where the pressure is leaking from

If the cylinder shows leakage, you’ll want to determine where the leak is. Possibilities include non-sealing piston rings or cylinder walls, leaking valves and combustion leaks. With the leak-down tester still attached to the vehicle, do the following:

• Listen for air coming from the oil fill hole: Remove the oil fill cap. If you hear air hissing out of the oil fill hole, pressure is leaking past the piston rings.
• Listen to the air intake: Air coming out of the intake indicates an intake valve is leaking.
• Listen to the exhaust pipe: If you hear air coming out of the exhaust pipe, an exhaust valve is leaking.
• Look for bubbles in the coolant: Remove the radiator cap. If you see bubbles in the coolant, combustion gases are leaking into the cooling system. This typically indicates a blown head gasket, cracked head or cracked engine block.

What to do if there’s a leak

An engine that fails a leak-down test has big internal problems. These issues should be handled by a professional repair shop.

Talk with a mechanic about the expected cost of the repair and figure out how much your car is worth. If the repair costs more than the car’s value, you might be better off getting a new vehicle.