Cylinder Leak Down Test - What Are The Results Telling You (2024)

Doing a cylinder leak down test, especially in conjunction with a compression test. Should allow you to quickly determine, the basic condition of any engine.

So, a compression test is a “dynamic test” (engine moving). On the other hand, a cylinder leak down test is a “static test” (engine at rest).

So, the compression test measures how much pressure, the engine can produce while cranking. In contrast, to the cylinder leak down test, which measures how much pressure, is lost in the engine.

Cylinder Leak Down Test - What Are The Results Telling You (2)

In a cylinder leak down test the engine is placed on top dead centre (TDC), of the cylinder in question. Then, using a similar type of connector as the compression test, we fill the cylinder with pressure. The tester then measures the volume of air needed to maintain, a predetermined pressure in the cylinder.

Firstly, the cylinder leak down test reading is shown in a percentage. Consequently, good cylinder leak down readings should be below 5-8%.

So, the great thing about the cylinder leak down test. It that it deals with how well the cylinder is sealing, and nothing else. Furthermore, the readings are not affected by, cam timing, or even engine cranking speed.

Another great feature of the cylinder leak down test. The fact that you can hear, where the air is leaking out of the cylinder. When a cylinder has high percentage of leakage, first check the oil filler cap. Do you hear a hissing sound? If so, you may have pressure leaking by the piston rings.

Cylinder Leak Down Test - What Are The Results Telling You (3)

Is there air leaking out the exhaust? Is it leaking out the intake system? Then a burned valve may be the problem. If two adjoining cylinders have similar low readings, and you hear leakage out the other cylinder. Then, a blown head gasket may be the problem.

So, being able to find the exact source of the compression loss. Will tell you where the problem is, and not just that you have one. Finally, this knowledge will greatly assist you in the next step… the repair.

This is my favorite engine test to do, because it can quickly point you in the right direction.

Cylinder Leak Down Test Findings – What Are The Results Telling You:


Air will escape past the valve, and into the intake manifold.

Cylinder Leak Down Test - What Are The Results Telling You (4)

Sound should be audible in the throttle plate, or air cleaner assembly. Be sure to have the (PCV) valve disconnected, so you don’t have a false positive.


Pressure will leak into the exhaust manifold, and ultimately out the exhaust. Will probably be more audible, than felt at the exhaust pipe.


Crankcase is pressurized, leading to air leaking through the (PCV) valve or breather. Possibly any other passage to the outside, like the dipstick tube. If the (PCV) valve is left hooked up to the intake, it could cause a false positive.


A cracked head might fail a cylinder leak down test, if it’s really, really badly damaged.

Cylinder Leak Down Test - What Are The Results Telling You (5)

Depending on the head design, it could leak into the water passage, leading to bubbling, audible in the radiator. Into the oil passage leading to noise at the (PCV), or another crankcase opening. To an intake or exhaust port leading to a sound, similar to a failed valve or seat. However, this is a less likely failure.


Typically a badly failed cylinder head gasket will be evident, when the head is removed. As a result, there will be evidence of combustion in the gasket, or on the mounting surfaces.


A badly scarred or gouged cylinder wall, could let pressure past the piston rings.

Cylinder Leak Down Test - What Are The Results Telling You (6)

Should sound the same as a bad piston ring. The only way to tell would be to remove the head, and physically inspect it.


Again, will show up the same way, as a cylinder wall or piston ring. Visual inspection would probably tell you.


Probably difficult to tell on a cylinder leak down test, unless pressure leaks past the fitting.


On DI engines. Should leak past the injector to the outside, or into the injector and into the fuel rail, theoretically.

To Look At It Another Way, In Order Of Likelihood:

  • Tailpipe – exhaust valve or seat, cylinder head gasket, cylinder head.
  • Throttle body, intake, or air cleaner – intake valve or seat, cylinder head gasket, cylinder head.
  • Radiator – cylinder head gasket, cylinder head.
  • (PCV), Breather, or other crankcase opening – piston rings, cylinder wall, piston, cylinder head gasket, cylinder head.
  • Outside – spark plug, cylinder head gasket leak, cylinder head, DI fuel injector.


Remember, start with simple, more likely failures, before assuming something catastrophic. Consequently, gaskets, valves/seats, and rings are all good places to start, depending on the symptoms. Finally, cracked heads, cracked pistons, and bad DI fuel injectors, are all pretty small possibilities. Finally, that’s why a cylinder leak down test, is a great test.


Cylinder Leak Down Test - What Are The Results Telling You (2024)


Cylinder Leak Down Test - What Are The Results Telling You? ›

No engine will have perfect sealing with 0 percent loss. Five to 10 percent loss indicates an engine in great to good running order. An engine between 10 and 20 percent can still run OK, but it'll be time to keep an eye (or ear) on things. Above 20 percent loss and it may be time for a teardown and rebuild.

What are acceptable leak down test results? ›

No engine will have perfect sealing with 0 percent loss. Five to 10 percent loss indicates an engine in great to good running order. An engine between 10 and 20 percent can still run OK, but it'll be time to keep an eye (or ear) on things. Above 20 percent loss and it may be time for a teardown and rebuild.

What can a cylinder leakage test tell you that a compression test can? ›

The leakdown tester is less common, and you need to get a good one to do the test properly. A compression test is to determine how much pressure the engine makes, and a leakdown test measures its ability to hold pressure.

What does a leak down test tell you on a 2 stroke? ›

If an engine is not running well, you can do a leak down test to determine both compression pressure (measured in psi) and the leak down rate. Compression testing alone doesn't always show the whole story, but leak down will show you how much compression each cylinder has and how well the cylinder is sealing.

What do compression test results mean? ›

A compression test reveals the condition of your engine's valves, its valve seats, and piston rings and whether these parts are wearing evenly. Healthy engines should have compression over 100 psi per cylinder, with no more than 10 percent variation between the highest and lowest readings.

How to read a cylinder leak down tester? ›

Leak down test results are typically represented as a percentage, indicating the amount of pressure that is lost from the cylinder. Ideally, you want to see a leak percentage under 10 percent, which could be noise or a slight ring or valve seal leak.

What is the range of leak test? ›

Setting the test time

Settings of a detection time of 2 to 10 seconds and a differential pressure value ΔP in the range of ±10 to 100 Pa are generally suitable.

Is a leak-down test better than a compression test? ›

A leak-down tester may represent a larger initial investment, and a leak-down test takes more time to perform, but it can give you a more accurate and detailed picture of the engine's overall health.

What three common defects can a compression test diagnose? ›

Diagnosing Potential Problems. A lack of compression is generally caused by 4 things – worn or damaged piston rings, damaged valves, improper valve timing, or a blown head gasket.

Will compression test show bad valves? ›

Place a compression tester in the spark plug socket, and crank the engine. The test will show if there are problems with the valves. You should also be aware of leaks from the exhaust pipe or the air inlet manifold.

What can a wet cylinder compression test tell us? ›

A compression check will tall you if there is damage to the cylinder, rings, or valves, basically telling you if the engine will compress and seal well enough to run. If you have contaminated fuel, the engine won't run properly or at all, but this water probably won't damage the cylinder.

What is a 4 stroke leak down test used for? ›

A Leak Down Test performed on a 4-Stroke engine is designed to check the percentage of cylinder pressure leakage. On a perfect engine leakage will be zero. (See chart below to help evaluate your results) This test is advantageous on evaluating your engines current performance status.

What are bad compression numbers? ›

But a good rule of thumb says that each cylinder in a mechanically sound engine should have compression of 130 psi or higher. While I've seen some people claim 100 psi is sufficient, the gearheads and other sources I've consulted consider that too low. In addition, you want consistency from one reading to the next.

Will a compression test show a bad head gasket? ›

Usually, a compression test won't show a problem with the head gasket.

How much psi for leak down test? ›

The difference in readings gives you the percentage leak-down. You'll need to prevent the crankshaft from turning, resisting 100 pounds per square inch normally used to test automotive gasoline engines. Diesel engines are typically tested at 200 psi. Small two-cycle and four-cycle engines might need only 50 psi.

How much valve leakage is acceptable? ›

The maximum permissible leakage rate shall be 40 milliliters per hour per inch of diameter of nominal valve size. (i.e. A 150NB (6”) Check Valve can have a permissible leakage rate of up to 240 mL liquid per hour.)

What is the limit for a leak test? ›

The leak test pressure limit for leak tests typically uses low pressure. Most codes for leak test pressure limits call for the pressure to be at least 15 psi or 25% of the design pressure (whichever pressure is less).

How do you calculate acceptable leak rate? ›

How to Calculate Leak Rate (SCCM)
  1. ATM = Atmospheric pressure (PSIA)
  2. V = Test volume (cm3)
  3. Δp = The decay in pressure during test time.
  4. Δt = The amount of decay time (min.)
  5. SCCM = Standard Cubic Centimeters per Minute.
  6. For example: Leak rate = .02psi/0.05min * 50cm3/14.7psia. Leak rate = 0.4 * 3.401. Leak rate = 1.36 SCCM.

What is the ISO standard for leak test? ›

ISO 20486:2017 specifies the calibration of those leaks that are used for the adjustment of leak detectors for the determination of leakage rate in everyday use. One type of calibration method is a comparison with a reference leak. In this way, the leaks used for routine use become traceable to a primary standard.

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