Oftentimes, when dealing with more advanced engine diagnostics, you’ll hear people talking about testing compression. It’s actually not a difficult process, and it can tell you a lot about what’s going on inside your engine. But, what is compression? Compression is the measure of the compressive force generated by the piston as is travels in the cylinder. There are figures specified by the factory that indicate whether or not your engine is still healthy. By comparing the results of your compression test to those numbers, you can learn a lot.

For a compression test, you need two things, a compression tester and a hot engine. The engine will need to be warmed up to operating temperature. If the car is drivable, the easiest way to accomplish this will be to take it on a short drive. If not, and the car will run, you’ll need to start it and let it idle until it comes up to temperature.

Once the car is warm, you’re going to want to shut it off and pop the hood. The engine will be hot, so be careful. Next, disable power to the car’s fuel pump, and disable power to the ignition coil. From there, pull the spark plug wires off all the plugs, and remove all the plugs. Mark the plugs or have some other way of knowing which cylinders they came from. Have a paper and pen handy. Screw the compression tester into the first cylinder you’re going to test. You’ll only need to thread it in hand tight.

Have a helper get in the car and crank the engine over six or seven times, and then stop. Take note of the compression tester’s behavior and reading. Did it rise slowly, or immediately. Was the compression within specification, or was it a lot lower than expected. Repeat the test for all cylinders. The best way to read the results is against a shop manual for your car. But, there are some generalities that apply everywhere.

If you have low compression in two adjacent cylinders, that’s usually a sign of a head gasket leak between them. This is often accompanied by overheating and other visually-identifiable symptoms of head gasket failure. Low compression across all cylinders often indicates worn cylinders or rings, and is usually an indicator that a full rebuild is in your near future. Low compression in a single cylinder can be indicative of several things. It can indicate a failed valve, usually an exhaust valve on that cylinder. It can also show head gasket failure, or even a cracked head. From there, you’ll want to have a leakdown test done. Unless you’re particularly well equipped, you’ll most likely have to take it to a shop for that.

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