Compression Check: Testing and Inspection

DIAGNOSTIC TESTING:

- The tests that follow evaluate engine condition--its ability to create and maintain compression. Careful analysis of the results can help to isolate causes of engine problems.

COMPRESSION TEST:

General:
- A test of compression pressures in the individual cylinders will tell a lot about the condition of the engine without the need for taking for taking it apart.
- The battery and starter must be capable of turning the engine at normal cranking speed to achieve meaningful results. The area around the spark plugs should be clean, to avoid getting debris inside the engine when they are removed. Because engine temperature may affect compression, the most accurate results are obtained when the engine is at normal operating temperature.

To Test Compression:

Hall Sender Connector & Power Output Stage Connector:

1. With the ignition off, disconnect the coil wire from the center of the distributor cap (terminal 4) and connect it to ground on the engine block using a jumper wire. This is to disable the secondary circuit of the ignition system.
2. Disconnect one end of the intake air duct from between the fuel metering unit (atop the air filter housing) and the throttle valve, to keep fuel from being injected during the test.
3. Remove the spark plug wires from the spark plugs. Use care to pull on both the wire and the boot at the same time to avoid damage to the connectors. Label the wires so that they can be reattached to the correct spark plugs. Remove the spark plugs.
4. Thread the compression gauge into the spark plug hole, just tight enough to seal. Over tightening will damage the seal on the gauge line.
5. Place the transmission in neutral. Hold the throttle wide open and crank the engine with the starter. Record the highest value indicated by the gauge.

NOTE: The gauge reading should increase with each engine revolution. Continue cranking until the gauge needle stops rising (about 4 to 5 revolutions).

6. Actuate the pressure release valve, or slowly unthread the gauge to relieve the pressure. Remove the gauge.
7. Repeat the test for each of the other cylinders. Record the data and compare with specifications.
8. Reinstall the spark plugs and wires. Reconnect the intake air duct. Reconnect the coil power output stage and distributor connectors.

Compression Specifications:
New: 9 to l2 bar (l31 to l74 psi)
Wear limit: 7.0 bar (102 psi)
Maximum difference between all cylinders: 3 bar (45 psi)

- Low compression is evidence of poorly sealed combustion chambers. The characteristics of the test results help isolate the cause or causes. Generally, compression pressures which are relatively even but below acceptable specifications indicate worn piston rings and/or cylinder walls. Low but erratic values tend to indicate valve leakage. Dramatic differences, such as acceptable values in some cylinders and very low values in one or two cylinders are the sign of a localized failure, probably of a head gasket. There are two more tests which can further isolate the problem.

WET COMPRESSION TEST:

- To analyze poor compression and further identify the source of the leakage, repeat the compression test, this time with about a tablespoon of oil squirted into each cylinder. The oil will temporarily help seal between the piston rings and the cylinder wall, practically eliminating leakage past the rings for a short time. If this test yields higher compression readings than the "dry" compression test, the difference can be attributed to leakage between the piston rings and cylinder walls, due either to wear or to broken piston rings. Little or no change in compression readings indicates other leakage, probably from the valves or a failed cylinder head gasket.

LEAK-DOWN TEST:

- The most conclusive diagnosis of low compression symptoms requires a leak-down test. Using a special tester and a supply of compressed air, each cylinder is pressurized. The rate at which the air leaks out of the cylinder, as well as the sound of the air escaping, can more accurately pinpoint the magnitude and source of the leakage. Any engine compression diagnosis which will require major disassembly should first be confirmed by the more accurate leak-down test. Because the test requires special equipment and experience, it may be desirable to have it performed by a Volkswagen dealer or other qualified repair shop.