Engine Compression Test

Make certain the battery is in good shape, as it will have to spin the engine quite a few times. Warm up the engine. The reason for this is that heat makes all the metal parts inside the cylinder expand and seal up better. Having things warmed up will give a more accurate compression reading. A cold engine will give inaccurate compression readings. Stop the engine, and remove all the spark plugs. This way the starter can spin the engine freely. Disable the ignition system by either pulling the coil wire, or disabling the ignition coil. To perform the actual test, insert the compression tester into one of the spark plug holes and crank the starter to rotate the engine in order to build compression in that cylinder. There may also be other required steps, such as disabling the fuel system or holding the throttle open. Record the compression number for that cylinder. Move onto the next cylinder until numbers have been recorded for each cylinder. Once all the numbers have been recorded they can be compared, and an overall conclusion can be drawn.

Even Burning
In a perfect world, the parts inside an engine all wear out together at the same rate. In every other world, this rarely happens. If your car or truck is using or burning a great deal of oil, has lost power, or is just plain running poorly despite a tune-up or other mechanical measures, a compression test is a good way to check what's going on inside the engine without taking it apart. Keep in mind that the numbers will mean nothing unless they are referenced against manufacturer recommendations found in a service manual. The thing to look for in a compression check is even numbers. If all the cylinders check out within 10 or so PSI of each other, and those numbers sync up with the factory specifications, then you're good to go.


If one or more of the cylinders show a difference of 15 or more PSI, then there are problems inside. If one cylinder shows a low reading, remove the compression tester and squirt a small amount of motor oil inside and test again. If the second test reveals a higher reading, then worn piston rings or cylinder walls may be the culprits. If the reading stays the same then suspect worn valves or valve seats. If the gauge shows a very low or no reading on any one or more of the cylinders, then serious internal damage has occurred. Any time a low compression reading is indicated on one or more cylinders, the time is right for engine work. There are a few tips to determining what these problems are, but keep in mind these are very general guidelines. The key point here is that an engine with low sealing compression in one or more cylinders will never run right—no matter how many new parts are connected to it.