A leak down test can tell you quite a bit more about an engine’s mechanical health than a compression test can. They both test the sealing ability of the combustion chamber, but the leak down can tell you how much is leaking and where it's going.
The procedure is similar to a compression test, to a point. I might break out my leak down tester after I find a cylinder with low compression, or I might start with a leak down tester when I suspect a mechanical problem with the engine. To perform the test, you must first remove the spark plug on the cylinder you're going to test. You can pull all the plugs to make turning the engine over easier.
In order to test a given cylinder, it needs to be at Top Dead Center (TDC). To get a cylinder at TDC, you need to turn the engine over by hand. This is much easier to do with all the spark plugs removed. As you might know, the piston goes to TDC twice during the four-stroke engine cycle: once for the compression stroke, and once for the exhaust stroke.
There are a few different ways you can determine this. The first is to follow the firing order of the engine, starting with cylinder one. To determine if you're on the compression stroke on cylinder one, you can use a couple of different techniques. The first is to place a piece of paper or other loose material over the spark plug hole as you rotate the engine. When the object moves out of the way, you know you're on the compression stroke. Another way is to install a vacuum gauge into the spark plug hole. You can actually use the hose that you thread into the spark plug hole and hook your vacuum gauge up to that.
I should mention that your vacuum gauge needs to be able to read pressure as well as vacuum. As you rotate the engine, look for the gauge to go into the positive. As the piston approaches TDC, the pressure will go to zero and then start to go to vacuum as you pass TDC. You can work the engine back and forth till you find the moment were TDC happens. Here's a video that explains the process.
Now that you know you have the piston at TDC, it's time to do the leak down test.
- Hook your tester hose directly into the spark plug hole.
- Hook your leak down tester up to shop air and zero the pressure. This will be important later so you know how much is leaking out.
- Then, hook the tester to the hose and take your reading.
Leakages above 20% are considered excessive. I like to see them closer to 10%, but as an engine wears, its compression and ability to seal will be lessened. But wait, there's more! Not only will you know how much is leaking, but you'll now be able to figure out where the pressure is going simply by listening for where the air is escaping.
- If you remove the oil cap and hear a lot of air escaping, the rings are worn and that's where your compression loss is.
- If you hear it coming out of the intake, the intake valve(s) isn't sealing.
- If you hear it coming out the tail pipe, the exhaust valve(s) is leaking.
- If you take the radiator cap off and see bubbles coming out, you have a combustion leak into the cooling system, which could be a bad head gasket.
I don't often use an actual leak down tester when doing this test. In fact, I do a compression test and look for a problem cylinder, and if I find it, I take a little short cut. You can actually use your compression tester hose for leak down testing. You first have to remove the Schrader valve in the compression tester hose. Once you've done that you can then hook shop air directly into the cylinder. You don't need to know how much is leaking at this point, because you’ve already determined that a particular cylinder has low compression. All you want to know now is where the leak is going.
Doing it this way is a personal preference, but it can save you from purchasing another tool, thus leaving a little cash in your pocket. If you've got mechanical issues with your engine, you're going to need it. Here's a video on leak down testing that can walk you through the process that includes this little trick of using the compression tester hose hooked up to shop air to find the source of a leak.