The Subaru Outback comes with either a 4-cylinder or a 6-cylinder engine. 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.
Begin by warming up the engine in the Subaru Outback, since running a compression test on a cold engine produces inaccurate results. Make sure that the battery is in good condition, since it will need to spin the engine several times.
Disable the ignition system to avoid the possibility of electrical shock and remove all of the spark plugs. If the Subaru Outback contains a 6-cylinder engine, there will be six spark plugs. If it has a 4-cylinder engine, there will be only four.
Insert the compression tester into one of the spark plug holes and crank the starter. This rotates the engine and builds compression in the cylinder. Write down the compression number and move on to the next hole. Repeat the compression test on each of the cylinders, keeping track of the results.
Check the compression numbers to see if they fall within 15 PSI of manufacturer specifications. If the Subaru Outback contains a 6-cylinder engine, the compression ratio is 10.7:1. Most models with a 2.5L 4-cylinder engine have a ratio of 10.0:1, but some vary so check the vehicle manual to be sure.
Compare the compression numbers to each other. The numbers should be similar and fall within 10 PSI of each other. If any cylinder has a low reading, add a little motor oil to it and run the compression test again. If the compression number remains the same, the cylinder has worn valves or valve seats. If it increases, the cylinder has worn walls or piston rings instead.
