The Nissan Altima originally came with a 4-cylinder engine but began offering the choice of a V6 engine with the 2002 model. As these engines become older, parts inside wear out and the car runs poorly, burns oil or loses power. A compression test is a helpful way of diagnosing these problems, such as worn cylinder walls, piston rings and valves.
Warm up the engine in the Nissan Altima prior to testing, since running a compression test on a cold engine will produce inaccurate results. The battery should be in good shape since it will need to spin the engine several times during the test.
Disable the ignition system to avoid electrical shock during the test and remove the spark plugs from the engine. If the car contains a V6 engine, there will be six spark plugs, but if it has a 4-cylinder engine, there will be only four spark plugs. Examine the spark plugs for oil or damage, since these are signs of problems within that cylinder.
Place the compression tester in one of the spark plug holes and crank the starter. This rotates the engine and builds compression in that cylinder. Write down the resulting number. Continue testing each of the remaining cylinders in the same fashion, recording the compression numbers. When you finish, there will be either four or six compression numbers, one for each cylinder.
Compare the compression numbers to manufacturer specifications. A Nissan Altima with a V6 engine has a compression ratio of 10.3:1, but for one with a 4-cylinder engine, the ratio varies by model year. If the model year is 1993 through 1999, the ratio is 9.4:1. For 2000 and 2001, the ratio changes to 9.2:1, and the ratio for 2002 through 2007 is 9.5:1.
Evaluate the test results. Compression numbers should fall within 15 PSI of the recommended compression ratio. Numbers should also fall within 10 PSI of each other. Retest any cylinders with low compression numbers.
Add a small amount of motor oil to the cylinder before performing the compression test again. If the number increases, the cylinder contains worn piston rings or cylinder walls. If the number is unchanged, it has worn valves or valve seats.
