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Engine Compression Test
By Mike Bumbeck/autoMedia.com
Printable version
Degree of DifficultyModerate
Moderate
Estimated Time180 minutes
180 minutes
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.
Prepare for the compression test by making sure the battery is up to snuff, warming up the engine, and disabling the ignition system.
Make Sure the Battery is Up to Snuff
Remove all spark plugs. Don't mix up the wires. Do not allow debris to fall into the cylinders. Determine which adapter will work for your engine.
Remove All Spark Plugs
Some kits come with threaded test fixtures. Do not over-torque the adapter.
Do Not Over-Torque the Adapter
Mount the gauge to the adapter. Hold the throttle plate open. Crank the engine over a few times until the needle on the gauge stops climbing. Record the final reading.
Mount the Gauge to the Adapter
Repeat the compression test for each cylinder. Record and analyze the final numbers.
Repeat the Test for Each Cylinder
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