Performing an engine leak down test can be daunting if you don’t understand the concepts and the theory behind them. It’s well worth learning however, this is a game changer when it comes to finding internal engine issues.
An engine leak down test can help identify various issues such as worn piston rings, leaking valves, or a blown head gasket. It works by pressurizing each cylinder and then measuring the amount of pressure lost, this test provides valuable insights into the overall condition and efficiency of the engine. It is particularly useful in diagnosing problems like low compression, misfires, or excessive oil consumption. Ultimately, a cylinder leak down test is your view into the heart of the engine.
Sounds complicated?… Don’t worry,I’ll explain what an engine leak down test is in simple terms. I’ll cover all you need to know about why this test is important, and when you need to perform one. I’ll explain it like you’re here at the shop with me.
Let’s get Started…
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What is an Engine Leak Down Test?
An engine leak down test involves pressurizing each cylinder and recording the amount of leakage. Simple enough right?
Why do it?
When a cylinder has too much leakage, compression can’t build up enough to prepare the air-fuel mixture for ignition, as a result, combustion can’t happen efficiently, or at all. This test can tell you if leakage is the problem, and help you find the cause.
The method is straightforward–introduce a known quantity of air pressure into the cylinder, and the gauge will indicate to you how much leakage there is.
What’s happening during an engine leak down Test?
An engine leak down test consists of setting a specific cylinder to “top dead center” of the compression stroke.
At this point, all of the valves are closed and the piston is at the top of its travel. This is the point when the air-fuel mixture is compressed and ignited in a running engine. Metered air is injected into the engine and the amount escaping the cylinder is measured.
By determining the amount of leakage, the mechanic is provided with valuable insights into the overall condition and efficiency of the engine.
Understanding combustion Leakage
During a compression stroke, the piston moves to TDC as all of the valves close. When the piston reaches the top of its stroke the valves and piston rings must hold back the cylinder pressure so the air and fuel mixture can be compressed, to allow it to be ignited properly.
Once the compressed air-fuel mixture is ignited, the piston is forced back down the cylinder, at the same time, the intake valve will begin to open, drawing in the next batch of air and fuel.
When the piston is traveling down the cylinder, it has a large amount of energy behind it from the burn of the air and fuel. This is what creates the “power stroke.” In essence, this is a short version of how engines run.
In order for the mixture to ignite it must be compressed to a minimum of somewhere around 120 psi, any lower than this and problems will start to develop.
Why Perform an Engine Leak-Down Test?
A leak-down test tells you how much compression is being lost within the cylinder based on a percentage. Once you know the cylinder has too much leakage you can start to find out why. The vehicle could have a burnt valve or even a scored cylinder, it does not matter, a leak down test will help you determine there is a mechanical problem and narrow down the reason why.
What happens when cylinders have too much Leakage?
If, for whatever reason there is too much cylinder leakage, the air-fuel mixture is not compressed enough and the combustion stops in that cylinder, causing a misfire.
In the case of oil consumption, compression may be mediocre at best, just enough to keep things running. However, the worn piston rings or failing valve seals are letting engine oil into the combustion chamber, where it is quickly burned.
What can a leak down test diagnose?
- A damaged exhaust valve
- Worn piston rings
- Scored cylinder walls
- Cracked cylinder head
- Major mechanical damage
- Bent valves. and more
What tools are needed to perform an engine leak down test?
You will need a leak-down tester, air compressor, spark plug hose adapter, basic hand tools, a long wooden dowel, a torque wrench, (for reassembly torquing) and rags. You will also need to find a way to turn the engine over by hand, usually, this can be done at the front crankshaft bolt, if you can’t get to the crank bolt you will need to get more creative.
How to perform an engine leak down Test
To start, get situated with all of the tools you need so you are not searching around for stuff when you start. I also recommend disconnecting the battery for safety reasons.
Set up on TDC
Grab the wooden dowel and put it down the spark plug hole of the cylinder you want to test, this will let us know where the piston is. Now slowly turn the engine over by hand and keep an eye on the dowel.
You will see the dowel move up or down, once you see it stick out as much as it will go the piston is on top dead center. It may take a few tries to find that sweet spot, but once you are close rock the engine back and forth to narrow it in.
Now that the piston is at TDC we know that we are either on the compression stroke or the exhaust stroke. it can only be one or the other.
Zero out the leak-down Tester Before beginning the leak-down test, ensure that the leak-down tester is properly calibrated and set to zero. This ensures accurate measurements during the testing process.
Leak down testers have two gauges, one for reading the pressure into the tool, and one for the percentage of leakage. The first thing we need to do is set the gauges so we can record the percentage of leaking air properly.
To reset the leakdown tester to zero, first, connect the spark plug bore hose to the tool and the compressed air source. It is important to note that initially, the hose should not be inserted into the spark plug hole while performing this step.
The air pressure is regulated so there is no need to worry about full shop air pressure coming out of the hose end.
Instead, plug the end of the hose off with your thumb and turn the air pressure regulator up or down to get The first gauge to read the pressure you want to put into the cylinder, (I always use 100 psi) and the leakage gauge to be right at zero when the hose is plugged off.
This zeros the leakage gauge to your initial pressure. Putting your thumb over the hose simulates ZERO leaks, when blocked off completely, the leakage gauge should be at exactly zero.
Get ready to start the Test
Stop right Here!
CRUCIAL REMINDER: It is essential to be aware that when compressed air is introduced into the engine during the compression stroke, it commonly causes the engine to rotate in the opposite direction. Therefore, it is crucial to prepare accordingly. Ensure that you are in a safe position and fully prepared before introducing air into the cylinder. Have someone assist you by holding the crankshaft using a breaker bar.
Install the adapter hose into the spark plug hole and tighten by hand, now hook the coupler end up to the leak down tester.
Connect the compressed air
Hook up the air source and have your helper hold the engine in place with the breaker bar and record the pressures. If you are on the exhaust stroke there will be huge leakage, this is because the exhaust valve will be open. A good way to check is to leave the air hooked up and feel for air coming out of the tailpipe.
If you are getting air out of the exhaust you know you need to turn the engine over one more revolution so the next time that piston is up, it will be on the compression stroke. If there is a problem with the exhaust valve you will feel this all of the time indicating an issue with the exhaust valve itself.
Evaluate the Data
Taking a look at the data is easy, remember how we set the gauges to 100 psi on the inlet and zero on the leakage gauge? Now that the other end isn’t blocked by your thumb the air goes into the sealed cylinder and the percentage of air leaking out of the cylinder can be measured.
The gauge will list the percentage of the leak.
To provide a point of reference, a leak-down rate of less than 5% is considered outstanding, while 10% is considered satisfactory and 15% is deemed acceptable. However, if the leak-down rate reaches 20%, it indicates there is a problem.
Two things to Note
- All engines have some leak-down, look at the numbers above, don’t go nuts if they are within range.
- There are adapters that bolt down to the engine block when the cylinder head is removed. This allows more accurate testing for cylinder wall scoring and piston ring wear.
Finding the source of the Leak
Depending on the numbers there may be a minor leak, or a large leak indicating mechanic failure. Remember that you are injecting air into the engine, you can listen for it.
Pull out the dipstick or remove the oil cap, can you feel the air pouring out excessively? That means combustion gases are entering the crankcase. This indicates worn piston rings.
Maybe you can hear the air in the intake, if that intake valve is closed there shouldn’t be, you may have a bent valve or another issue.
Hopefully, you are starting to see the value of this important test. It will allow you to determine if there is a problem to chase, and it can show you exactly what’s causing it.
Performing an engine leak down test is a valuable diagnostic tool that can help identify potential issues with your engine. By following the steps outlined in this blog, you can easily perform a leak down test, evaluate the results, and make your next move.
Whether you’re a seasoned mechanic or a car enthusiast looking to troubleshoot engine problems, this test can provide valuable insights into the health of your engine or shine a light on a bigger issue.
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