A compression test is a helpful tool used to diagnose engine trouble in a VW Jetta. It is easy to perform and doesn't require tearing apart the engine to see what isn't working properly inside of it. Some common problems revealed by this simple test include worn valves, piston rings and cylinder walls.
Warm up the engine in the VW Jetta since a compression test ran on a cold engine will not be accurate. Be sure that the battery in the car is in good condition since it will need to spin the engine several times.
Turn off the car and remove all of the spark plugs from the engine; the number varies depending upon engine size. Now the starter can spin the engine freely. Disable the ignition system to avoid electrical shock during the compression test.
Place the compression tester into one of the spark plug holes and crank the starter to rotate the engine. This builds compression in that cylinder. Record the resulting compression number for comparison later. Repeat this process for each of the cylinders, keeping a record of all the compression numbers.
Check to see if the compression numbers fall within 15 PSI of manufacturer specifications for the VW Jetta. If the car contains a five-cylinder engine, an ideal reading is 9.5:1. In models with a four-cylinder engine, the recommended compression ratio varies by engine. If the car contains a 2.0L engine, the ratio is 10.3:1 but if it has a 1.9L engine, the ratio is 18.5:1 instead.
Compare your numbers to each other as well. Each number should fall within 10 PSI of the others. If one cylinder shows a low reading, add a small amount of motor oil to that cylinder and perform the compression test again. If the compression number increases, worn cylinder walls or piston rings are the problem. If the number doesn't change, the engine has worn valves or valve seats instead.
