2. Warm up the engine to normal operating temperature. The battery must be fully charged. All spark plugs must be removed. An assistant is required. If necessary, remove the top engine cover.
3. On engines with a distributor, disable the system by disconnecting the central high-voltage wire from the distributor cap and grounding it to the cylinder block. Use a "cigarette lighter" wire to ensure a secure connection. On engines without a distributor, disconnect the wiring from the ignition unit (see chapter 5B).
4. Disconnect the wiring connector from all fuel injectors.
5. Attach a compression tester to the free hole of the first cylinder - it is preferable to use a compression tester with a screw-in tip.
6. Ask an assistant to fully depress the accelerator pedal.
Note: Some later models are equipped with an accelerator pedal position sensor instead of a cable. The throttle will not open until the ignition is turned on.
Turn the engine over with the starter for a few seconds, after two or three engine revolutions the compression value will increase to the maximum and should then stabilize. Write down the highest value obtained.
7. Repeat the measurement on the remaining cylinders, recording the values obtained. Keep the throttle open at all times.
8. The cylinders should show the closest possible compression values. A difference of more than 3 bar between two cylinders indicates a malfunction. Compression on a serviceable engine should increase quickly. Low compression on the first compression stroke, gradually increasing on subsequent strokes, indicates worn piston rings. A low value on the first stroke, not increasing on subsequent ones, indicates a loose fit of the valves in the seats or a broken cylinder head gasket (a crack in the cylinder head can also be the cause).
10. Compare the obtained values with the data given in Specifications.
10. If the compression in any cylinder is less than the prescribed value, perform the following test to determine the cause. Pour a teaspoon of clean motor oil into the spark plug hole and repeat the compression test.
11. If adding oil temporarily increases the compression reading, this indicates excessive cylinder or piston wear. If the compression does not increase, this indicates a burnt valve, or a blown head gasket may be to blame.
12. Low compression values in adjacent cylinders are almost always a consequence of a breakdown of the gasket between the cylinders.
13. If the compression in one cylinder is less than in the other by about 20 percent and the engine is "slightly feverish" at idle, the cause may be worn camshaft lobes.
14. After completing the check, tighten the spark plugs, connect the high-voltage wires to them, restore the ignition wiring and install the top engine cover.
[The article is a reprint of material from: AudiManual]