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How to Check Cylinder Compression




Extremely rough idle. Exhaust smoke under load. Overheating. These are all situations that no car enthusiast wants to experience. But instead of replacing parts in a hope to find a resolution, one simple check of cylinder compression check can narrow down your options down in a hurry and eliminate the guesswork of a blown head gasket, worn-out rings or other situation. So it’s time to roll up those sleeves and learn a simple trick of the trade called the compression test.


But first, you’ll need a test gauge. There are two types to choose from with the first being a simple compression test gauge (1), which is just a pressure gauge and rubber hose with various fittings for different spark plug thread sizes. The more complex is the leak-down compression tester, which includes two in-line gauges with hoses attached on both ends (2). Most auto parts stores will rent a gauge, but a basic compression gauge is not too terribly expensive to own.


To start your cylinder compression check, first, you’ll need to unplug the ignition system to keep the motor from starting up. Unplugging the ignition module or spark plug wire leading to the distributor cap should work. Next, remove a cylinder plug, then thread in the compression gauge spark plug hose attachment and snug up the fitting. Lastly attach the compression gauge to the hose and your ready to crank over the motor and get a reading.


Usually you’ll want to let the motor crank for about six times to get a full compression reading. This process will need to be consistent when checking the other cylinders to keep your compression figures more accurately from cylinder to cylinder. Also perform the check three times to make sure you have an average figure from all three readings and record the most consistent figure for each cylinder, and each cylinder should have about same exact reading.



The reading of compression can range quite differently between various engines and combinations, so you'll want to do a bit of research to find where you should be at.  Overall, If you notice a drop in one cylinder figure around 10-20 pounds, this could indicate a potential issue and will need to be evaluated further with a complete leak down test. If your variance is no higher than 5-6 pounds, this can just be normal cylinder or ring wear and usually will not result major running issues.


So that’s it. If you perform this and have no serious issues, you should be good to move along, but if you see a large variance in a particular cylinder, the next step is a leak down -- which we may cover in another article. Although compression testing is basic start, it’s one that could save a lot of time and money in the long run.






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