The Chevy Cobalt comes with a standard 4-cylinder engine in three sizes, 2.0 L, 2.2 L and 2.4 L. When valves, piston rings and cylinder walls wear out, the engine loses power, burns oil or runs poorly. A compression test is a helpful tool used to diagnose these problems without tearing apart the engine.
Turn on the engine in the Chevy Cobalt and let it warm up since performing a compression test on a cold engine will provide inaccurate results. The battery must be in good condition for the test. Once the engine is warm, turn it off before proceeding with the test.
Remove the spark plugs and disable the ignition system by pulling the coil wire or disabling the ignition coil. Be sure to do this before testing to avoid electrical shock.
Insert the compression tester into one of the four spark plug holes and crank the starter to rotate the engine. After building compression in the cylinder, write down the compression number. Do the same for the other three cylinders in the Chevy Cobalt.
Compare the four compression numbers to manufacturer specifications as well as each other. The compression ratio for the engine of a Chevy Cobalt depends on the engine size. It is 9.5:1 for a 2.0 L engine, 10:01 for a 2.2 L engine and 10.4:1 for a 2.4 L engine. Ideal readings fall within 15 psi of the recommended number and within 10 psi of each other.
Repeat the compression test on any of the cylinders showing a low reading. However, add a capful of motor oil to the cylinder before doing so. If this procedure results in a higher test number, then you have worn cylinder walls or piston rings. If the number doesn't change, then you have worn valves or valve seats.
