- 1). Warm up the engine in the VW Passat since a compression test ran on a cold engine will be inaccurate. Be sure that the car battery is in good condition since it will need to spin the engine several times.
- 2). Turn off the engine and remove the spark plugs. If the car contains a four-cylinder engine, there will be four spark plugs, but if it has a V6, there will be six instead. Now the starter can spin the engine freely. Disable the ignition system by either pulling the coil wire or disabling the ignition coil.
- 3). Place the compression tester in one of the spark plug holes. Crank the starter to rotate the engine and build compression in that cylinder. Record the compression number.
- 4). Repeat this process for each of the spark plug holes, keeping a record of the compression number for each cylinder. Compare these numbers against the recommended compression ratio to see if they fall within 15 PSI. If the VW Passat is a 2006 through 2008 model and contains a four-cylinder engine, the compression ratio is 10.3:1, but if the car has a V6, the ratio is 12.0:1.
- 5). Compare the numbers against each other. Ideal readings will be within 10 PSI of each other. If one or more cylinders show a low reading, add a tiny amount of motor oil and test again.
- 6). Evaluate the new readings. If the new number is higher, worn piston rings or cylinder walls are present. If the number is the same, worn valves or valve seats are the problem. A very low reading or none at all on one or more cylinders is a sign that serious internal damage has occurred.