Which is the best test to use?
To use a leak down test or compression test, which one is going to give you the best indication of what’s going on in the engine? Both a leak down test and compression test are ways to get a picture of the condition of the combustion chamber of your engine. However, one test is far better than the other
A Compression Test
A compression test is exactly as it sounds, it measures the amount of pressure that an engine can generate while cranking. The way that this test works is that all the spark plugs are removed from the engine. Then install a compression gauge in place of one of the spark plugs. Then crank the engine over using the starter. The compression gauge will record the amount of pressure generated by the engine, usually in PSI. Once one cylinder has been measured the gauge is moved to the next cylinder. Repeat the procedure on all cylinders.
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, then set the cylinder that 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 air supply and will put air pressure into the cylinder to test it. Once one cylinder has been tested the gauge is moved to the next cylinder. Repeat the test until all cylinders have been measured.
Pros and cons of Compression Testing
Pros, doing a compression test is a well-known and accepted test method. Most mechanics will have a compression tester in their toolbox and are used to using it. There are several styles of testers. Both the screw in type as well as a hand-held type that makes for a very quick test. As the tester is just held against the spark plug hole while the test is performed. Lastly, the test can be performed anywhere as it does not require a source of compressed air.
Cons, a compression test if done improperly will give you a false result indicating a low compression. If the vehicle 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 held in the wide-open position then the test will show a false low reading.
When a true low compression reading is given the gauge needs to be removed and then oil added to the cylinder and the test redone. This is an attempt to locate the source of the low compression. A high compression reading after adding oil to the cylinder indicates worn piston rings. This is also known as a “Wet Compression Test”.
Best Compression Testers
Pros and Cons of a Leak Down Test
Pros, when using a leak down test the engine does not need to be cranked over using the starter motor. This means the test can be done without relying on other systems than can produce a false reading. Because it doesn’t require a starter to crank the engine, an engine can also be tested while out of the car.
This is great when checking a used engine or an unknown replacement engine. The leak down test is always conclusive. The source of the combustion leak is always known by the end of the test. If an intake valve is leaking air pressure can be felt coming through the intake system. If an exhaust valve is leaking then air will be felt or heard coming out the tailpipe. When the rings are leaking then air will come out of the oil filler cap. 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.
Leak down test requires an air compressor
Cons, the leak-down tester needs compressed air to work. Depending on the size of the compressor being used it may not be as accurate as when the test is being performed in a shop. The test takes longer to perform. Because the engine needs to be set at TDC compression for each cylinder it just takes more time. Sometimes it can be hard to correctly locate TDC on all cylinders. If the cylinder is not right at TDC it will rotate when air pressure is applied to the cylinder. This means resetting the cylinder back to TDC and starting the test again.
Best Leak Down Testers
So, which is Best?
For me, the leak down test far outweighs the time savings of performing a compression test. A compression test is subjective to cranking speed and temperature. So its results just don’t give a good enough picture of the combustion chamber condition to make an informed decision. If a true low reading is given, usually anything less than 100 PSI. There is still more work to find the actual source of the leak. For example rings, or valves.
With a leak down test, I know straight away if there is an issue or not. And if there is an issue I know exactly where to go to fix the problem. Even though the set-up time is a little longer, I know that I will have meaningful results that can be acted upon. So, if you are trying to decide on what test to use, my advice is to go with the leak down test.