What is a leak down test? And why is it better?

Which is the best test to use? A leak down test or compression Test

When it comes to accessing the internal condition of your engine, there are two different types of tests that can be done. One is a leak down test and the other a compression test. Both of these tests will give you an idea about the internal condition of the engine.

A Compression Test

A compression test is exactly as it sounds. It measures the amount of pressure that an engine can generate while cranking. To perform a compression test, remove the spark plugs from the engine. Then a compression gauge is installed in place of one of the spark plugs. Then the engine is spun using the car’s starter motor. The compression gauge will record the amount of pressure generated by the cylinder, usually in PSI. Once one cylinder has been measured, the gauge is moved to the next cylinder, and the test is repeated until all cylinders are measured.

Compression Tester Kit

A Leak down Test

A leak-down test works by putting air pressure into the cylinder, and the gauge measures the amount of pressure drop or leakage in the cylinder. First, you must remove the spark plug and then set the cylinder you wish to test to TDC compression (Top Dead Center of the compression stroke). Then install the leak down test adaptor and connect to the gauge. The leak down tester is connected to the shop’s air compressor, and the gauges are calibrated or zeroed. The leak down tester measures the amount of air lost from the cylinder. Once one cylinder has been tested, the gauge is moved to the next cylinder, and the test is repeated until all cylinders have been tested.

SnapOn Leak down Tester, leak down test

Pros and cons of Compression Testing

Pros of a compression test

Doing a compression test is a well-known and accepted test method. Most mechanics have a compression tester in their toolbox. Most mechanics know how to use the compression tester.

There are several styles of compression testers. Screw-in compressions gauges use the spark plug threads to connect to the cylinder. Handheld versions have a tapered rubber seal. You just press them over the open spark plug hole with your hand.

One of the biggest pros is, you do not need an air compressor.

Cons of a compression test

A compression test, if performed improperly, will give you a false result indicating low compression. If the vehicle’s battery or starter motor is defective, resulting in a low cranking speed, then the compression test will be false and most likely show low compression. If the throttle is not in the wide-open position, then the test will show a false low reading. When you get a low compression reading you need to retest. Remover the gauge, add oil to the cylinder and perform the test again to locate the source of the low compression. A high compression reading after adding oil to the cylinder indicates worn piston rings. When you add oil, the test is a “Wet Compression Test.”

Pros and Cons of a Leak Down Test

Pros of a Leak-down test

The engine does not need to be spun using the starter motor when using a leak-down test. This test can be done without relying on other systems that can produce a false reading. Also, because a leak-down test doesn’t require a starter to crank the engine, the engine can be tested while out of the car. This is great when checking a used engine or an unknown replacement engine.

A leak down test is more conclusive. The reason for the combustion leak is clear by the end of the test. When an intake valve has a leak air pressure can be felt coming through the intake system.

If an exhaust valve is leaking, air will be felt or heard coming out of the tailpipe. If the piston rings are leaking, air will come out of the oil filler cap or breather. As a side note, there will always be some air pressure coming out of the crankcase as piston rings will never have a 100% seal.

Cons of a Leak-down test

A leak-down tester needs compressed air to work. The test takes longer to perform because the engine is set at TDC compression for each cylinder. Sometimes it can be hard to locate TDC on all cylinders correctly. If the cylinder you test is not at TDC, it could rotate under air pressure. This means resetting the cylinder back to TDC and starting the test again.

So, which is the Best Test to use, the Leak Down Test or Compression Test?

The leak down test far outweighs the time savings of performing a compression test. Compression tests can be very unreliable. A compression test also relies heavily on the car’s battery and starter motor. A bad battery or starter motor will produce a low cranking speed, resulting in a low compression reading. If you do not hold the car’s throttle wide open, the compression test will not be accurate and produce a low compression number. If a compression test shows a low number, it will require further testing to identify the cause of the issue.

The leak down test is a more accurate form of test. It will identify the internal condition of the combustion very quickly. If there is a combustion leak, you do not need further testing to find the actual source of the problem. Simply listening for where the air is leaking out will tell you what the problem is.

Buying a Leak Down Tester

A leak down tester will cost about the same as a compression tester. However, there are cheap versions of both tools on the market. When buying a leak-down tester, spending a little more and getting a better tool that will last longer is better. Cheaply made leak down testers will often use low-pressure gauges and cheap rubber hoses. Using a low-pressure gauge will mean that you need to regulate your shop air pressure down to use it.

Combination of different leak down testers

Different cars and trucks use different spark plug sizes and threads. Ensure that the leak-down tester you buy has the right adaptor available for the model of engine you are testing. Spending a little more money on a quality tool will usually include multiple test adaptors with it.

Using a leak down tester and what results you can expect to see

Before using the leak down tester, you should read the included instructions. You will usually need to calibrate the tester to your air supply.

To test a cylinder:

  1. Remove the spark plugs and bring the cylinder up to TDC and install the adaptor hose.
  2. When you attach the leak down tester, make sure that the engine does not rotate.
  3. If the crankshaft does turn, stop the test, and reset the cylinder to TDC.

Sometimes you may need to lower the testing pressure to stop the crankshaft from turning.

No cylinder will have a perfect seal, and you can expect to see some small leakage from even the best cylinder. As a rule of thumb, a leak-down value of 5% to 15% is normal. An engine in excellent condition will be closer to 5 to 10 % leakage. Leakage numbers of 15 to 30% mean that there is significant wear occurring. Depending on where the leak comes from, you may need to repair it soon. Cylinder leakage above 30% will require more immediate attention.

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