Engine cylinder leak down testing explained + Pro Tips (2024)

Engine cylinder leak down testing allows mechanics to pinpoint the cause of compression loss in a specific cylinder. A cylinder leak down test gives mechanics an inside look at the internal integrity of any engine, finding many issues that would otherwise take some major disassembly to pin point.

  • Burned valves
  • Worn piston rings
  • Blown head gasket
  • Jumped camshaft timing
  • Bent valves
  • Much more…

This straightforward test can diagnose sporadic misfires, excessive oil consumption, and many other internal engine issues.

The two-fold advantage of an engine cylinder leakdown test is you can measure engine cylinder leakage while determining where the weak link is all at the same time.In this lesson, I will explain an engine cylinder leakdown test in depth. I’ll explain how the test is done and where to look for problems.

A scenario that requires a cylinder leak-down test…

You’re working on a cylinder misfire and have checked ignition and fuel delivery, along with all of the other typical culprits. Everything is OK but the misfire remains… Either intermittent or consistent. Checking cylinder compression, you have determined low compression on the weak cylinder … We now know the misfire is due to a lack of compression and not a faulty ignition coil or fuel injector.

You have done the initial steps to diagnose the engine misfire, confirming low cylinder compression is the issue.

What’s next?.. This is when performing an engine cylinder leak down test will determine where the compression is being lost

Let’s get started…

What’s the engine cylinder leak down test?

Low compression on one or more cylinders indicates a definite issue, but what is it?A cylinder leak down test will tell you, let’s break it down.

Imagine you are working on a 4-cylinder engine that has a hard misfire on cylinder #3. You have done basic misfire diagnoses and a full compression test on all cylinders.

Here are the results…

  • Cylinder #1 = 155psi
  • Cylinder #2 = 160psi
  • Cylinder #3 = 65
  • Cylinder #4 = 160

Clearly, cylinder #3 has seen better days, it’s well under the 130 psi threshold. This compression test has shown the engine has a definite issue in cylinder #3 .. but what? It’s not like your customer isn’t going to ask you what the cause of the problem is after you explain their lack of compression in the engine.

an engine cylinder leak down test will show you, WHERE the problem is.

Introducing compressed air into the cylinder and measuring the amount of leakage will immediately show you the integrity of the cylinder. This is a way to determine if the low compression is caused by worn piston rings, a burnt valve, a cracked head, or other major damage.

Being able to measure pressure loss accurately is key. Not all cylinders have major loss, such as a hole in a piston, or a burnt valve. There may be a cylinder that is borderline due to worn piston rings or another issue that has not quite yet become obvious.

Issues like this are notorious for setting intermittent check engine lights and driving you and the customer crazy in the process. Any average mechanic can perform a quick compression test and determine if the cylinder has low compression.

The technician who understands a cylinder leak down test can accurately tell you WHY the cylinder has low compression in the first place.

How does a cylinder leakdown test work exactly?

A leak down tester consists of two gauges, one for pressure in (baseline) and one for pressure out (the cylinder itself.)

For example, if you set the tester up at 100psi and plug the outlet off with your finger BOTH gauges will read at 100psi.

This is because there is NO LEAKAGE, 100 is 100 psi. Once the end you have plugged is connected to the cylinder to be tested using a hose and the spark plug bore, 100 psi is injected into the cylinder.

100psi is still the baseline pressure, however as the cylinder leaks off the air pressure the second gauge reads lower. This is the leak down of the cylinder. If the cylinder were 100% sealed with absolutely zero leakage both gauges would read 100psi.

In this case, if the second gauge read 90 psi this would indicate a 10% leakage.

10% of 100 = 10.

100 – 10 = 90.

The cylinder is unable to hold 100 because the air is escaping somewhere, exactly as it does when the engine is running.

It’s important to note no engine has zero leakage, there will always be some blowby past the piston rings. Anything over 10% leakage indicates problems are starting.

Reasons You Need to Know How To Perform an Engine Cylinder Leak Down Test

Engine cylinder leak down testing explained + Pro Tips (1)

Diagnosing today’s cars requires accurate and complete testing. Simply doing a compression test doesn’t fully diagnose the problem, it only shows the result.

To advise your customer or service writer as to what the actual cause of the problem is you need to know how to perform this test and be able to filter the result. If you are serious about being a technician tests like this will separate you from the rest of the pack.

What a leak down test can reveal…

  • Burnt intake valve
  • Burnt exhaust valve
  • Worn piston rings
  • Blown head gasket
  • Valves out of adjustment (too tight)
  • Overall cylinder condition
  • Major damage (hole in piston, scored cylinder walls)

How to perform an engine cylinder leak down test

An engine cylinder leak down test is straightforward as long as you understand what you are doing. Let’s break down the meat and potatoes of it.

The key is understanding what stroke the cylinder to be tested is on. We are looking for TDC, or “top dead center” of the COMPRESSION STROKE. TDC of the compression stroke is when the piston is at the top of its stroke and all the valves are CLOSED, creating a sealed combustion chamber.

This is where the compressed air and fuel mixture feel the squeeze before the spark plug ignites it.

Using a leak down tester…

Engine cylinder leak down testing explained + Pro Tips (2)

Step 1: Get the tools you need

We are gonna need some stuff to get this test done…

  • A socket and ratchet to rotate the engine by hand
  • A decent engine cylinder leak down tester
  • The correct hose for the tester
  • A compressed air source (an air compressor that can keep up)
  • A spark plug socket and normal hand tools

More advanced tools (optional)

  • A CEM meter (for measuring very slight pressure increases)
  • A bore scope (for viewing piston position within the cylinder)

Step 2: Prepare the car

Engine cylinder leak down testing explained + Pro Tips (3)

If possible run the car for a bit to get the cylinders warm and oil around the piston rings. This will give us accurate results.

I need to say the boring stuff here, when you are doing this test you should have the battery disconnected and take all of the necessary safety measures to avoid getting hurt or damaging a vehicle.

With the safety talk out of the way, let’s get access to the one cylinder we want to test. Remove the coils or wires to access and remove the spark plug.

Very important!

When the cylinder is pressurized with air at TDC of the compression stroke the engine WILL turn over as the compressed air pushes against the piston. Depending on how well sealed the combustion chamber is it can be pretty quick.

You will need a helper to hold the crankshaft with a breaker bar as you do the test. The pressure isn’t insane, but it is enough to hurt you badly if a finger or arm is in the wrong place at the wrong time. A good grip with a breaker bar is more than enough to hold it back.

Step 3: Find Top Dead Center

There are a few ways to find TDC, the first and easiest way is to put a small wooden dowel into the cylinder through the spark plug hole.

Use the breaker bar to turn the engine over slowly. You should see the dowel lift or drop. When the dowel raises fully the piston is on TDC. You will need to turn the engine back and forth a little to find the sweet spot.

Once the dowel reaches max lift the piston is all of the way up within the cylinder, or TDC.

Now if you are familiar with the 4 strokes of a typical gas engine you know that with the piston up in the cylinder, you are either on the exhaust stroke or the compression stroke.

Step 4: Hooking up the leak-down tester

Now that the piston is on TDC let’s calibrate the setup.

Attach the test hose to the leakdown tester and attach the other end to your air source. Air should be blowing out of the test hose end at fairly low pressure, with a slight hiss indicating leakage. Plug off the end of the hose using your thumb and adjust the air pressure regulator until the second gauge reads 100 psi, (or max depending on the gauge.)

You are adjusting the inlet air pressure so the gauge measuring the cylinder pressure is at 100 psi when there is ZERO leakage.

If the cylinder you are testing had absolutely no leakage the gauge would read 100 psi. This gives us a reference point so we can accurately measure the leakage in the cylinder.

Once the gauges are set screw the test hose into the cylinder spark plug hole, (a leakdown tester has the same style of hose as a compression tester.)

Step 5: Perform the test

Attach the engine cylinder leak-down tester and air source. Don’t forget that the engine WILL most likely rotate back if you are on the compression stroke unless there is a huge leak.

I usually add air and watch the crankshaft pulley for the first run. If I am on the compression stroke the engine should spin, or at least try.

Turn the engine over by hand in the direction of rotation until the piston is at top dead center (TDC on the compression stroke, using the wooden dowel.)

Before adding air into the cylinder make sure you have a long breaker bar on the crankshaft bolt and a helper to keep the engine from spinning. If the car has a manual transmission put the car in gear and set the parking brake. This will stop the engine from turning.

Hook up the air supply to begin injecting air into the engine and record the leak down. If the leak down is over 10% it’s time to start looking for where the pressure is escaping.

How to Pinpoint the Cause Of High Cylinder Leakage

If you are seeing high leakage within the cylinder (over 20%) it’s time to start looking at where the pressure is escaping.

These are a few places to check for leakage…

  • The rear exhaust: Leaking air indicates an exhaust valve issue.
  • The intake manifold: Air felt or heard from the intake manifold indicates an intake valve issue.
  • The cooling system: Bubbling in the radiator or coolant reservoir indicates a bad head gasket or cracked cylinder head.
  • The dipstick tube or oil cap: Air coming from either indicates engine blow-by, worn piston rings, or a scored cylinder.

Start by removing the engine oil dipstick. If you feel slight air escaping the dipstick tube this is an indication of engine blow-by. Blow-by is a condition where the piston rings are not sealing properly. If you feel or hear air coming out of the crankcase chances are the piston rings have seen better days. This can be caused by worn piston rings or serious engine damage, ie a scored cylinder wall etc.

There are instances when the air escaping is very light. This is when using a smoke machine can help.

If you have a high leak down (20% and over) add a small amount of clean engine oil to the cylinder through the spark plug hole. Engine oil is required to help the rings seal and keep blow-by to a minimum. If the cylinder is “washed down” meaning fuel has washed down engine oil on the rings, you will get high cylinder leak-down readings.

Key Points For a Successful Engine Cylinder Leak Down Test

An engine cylinder leak down tester is a must-have tool for any serious mechanic. This test will allow you to pinpoint internal engine issues… Misfires, excessive oil consumption, and power loss are a few of the primary culprits.

a few key tips to help you perform an engine cylinder leak down test

  • Cylinder leakage is normal to a certain point, anything over 10% leak down may indicate issues. 15 to 20% and higher is a definite problem.
  • Remember the engine WILL usually turn over when air is injected, it’s not uncontrollable, so be prepared.
  • Add clean engine oil to the cylinder if it reads high leak down.
  • Feel and listen to where the air is going, remember the point of this test is to find out where cylinder pressure is escaping.
  • Always zero out the reading gauge by plugging the hose end off and adjusting the air regulator until the gauge reads 100% even with no leakage.

Wrapping Up engine cylinder leak down testing

I’ve been a mechanic for close to 30 years. Over that time I have diagnosed countless engine issues by performing the engine cylinder leak down test. This test is simply the best way to determine the cause of low compression and weird intermittent misfires.

Remember to confirm you are on TDC of the compression stroke before testing. If you feel airflow out of the exhaust chances are you are on the exhaust stroke.

Spin the engine over a revolution and try again, if you still feel air at the exhaust pipe a burnt, bent, or stuck exhaust valve is something to look for.

A decent bore scope is a great way to take a look inside of the cylinder to check for visible damage after the leak down test is performed.

Engine cylinder leak down testing explained + Pro Tips (4)

Jerry Petrillo

Jerry Petrillo is a dad, mechanic, and blogger with over 25 years of hands on automotive business experience. He holds many active certificates and licenses, including ASE certification, Hunter engineering, and Bosch electrical.

Engine cylinder leak down testing explained + Pro Tips (2024)
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