Compression check (Audi A6 C4)

If the engine runs poorly or intermittently, check the ignition system and fuel system. If the reasons for poor engine performance cannot be found, perform a compression test. Regularly performing this test will also allow you to prevent malfunctions in advance.

To perform the check, the engine must be warm, the battery must be charged, and the spark plugs must be unscrewed.

Execution order

1. Disable the ignition system by disconnecting the ignition coil high tension wire from the distributor cap. Ground the wire to the cylinder block.

2. Connect a compression tester to the spark plug hole of cylinder N1.

3. Open the throttle valve fully and turn the engine crankshaft with the starter. After one or two revolutions, the compression pressure should increase to the maximum mark and then stabilize. Write down the result.

4. Perform a similar check on the remaining cylinders and record the readings obtained.

5. The pressure in all cylinders should be the same. A difference of more than 2 bar between any two cylinders indicates a malfunction. In a healthy engine, the pressure should build up quickly. Low first-stroke pressure that gradually builds up on subsequent strokes indicates worn piston rings. Low first-stroke pressure that does not build up on subsequent strokes indicates a valve leak or a blown head gasket. Deposits on the bottom of the valves can also cause low compression.

6. If you have any doubts during the inspection, please contact the specialists.

7. If the pressure in any cylinder is too low, perform the following test to determine the cause. Pour a teaspoon of clean motor oil into the cylinder through the spark plug hole and repeat the pressure test.



8. If the compression pressure increases after adding oil, it means the piston or cylinder is worn. If the pressure does not increase, it means the valves are worn or burnt, or the cylinder head gasket is broken.

9. Low pressure in two adjacent cylinders almost always means that the head gasket between them is blown.

10. If one of the cylinders is about 20% lower in pressure than the others, and the engine is not idling properly, it means that the cause may be a worn camshaft cam.

11. After completing the check, screw the spark plugs back into place and connect the ignition system.

[This publication is borrowed from the resource AUDIMANUAL]