Warning: For diesel engines, a compression tester with a measurement range of approximately 40 atm is required.
The engine must be warmed up to operating temperature, the battery must be fully charged. You will also need the help of an assistant.
The maximum permissible pressure difference in individual cylinders can be 3.0 atm. (for diesel engines 5.0 atm.).
Compression pressure in a healthy engine increases very quickly. A low reading after the first cycle, increasing with subsequent cycles, indicates worn piston rings. A low reading after the first cycle, not increasing after subsequent cycles, indicates either leaky valves or a blown head gasket (the cause could also be a crack in the head). The presence of carbon deposits on the valve plates can lead to a decrease in compression.
It should be noted that due to different starter speeds on different models, the results obtained from measurements may differ. The results obtained from compression measurements should be approximately the same for all cylinders.
If the pressure in any cylinder is at the minimum permissible level or even lower, then to find out the reason, do the following: Pour a teaspoon of engine oil into the cylinder through the spark plug hole and repeat the compression measurement.
If adding oil temporarily improves compression, the reason for its decrease is most likely wear of the piston, rings or cylinder. If there is no increase in compression, then it can be assumed that the cause is leaky valves or a broken head gasket.
Low compression in two adjacent cylinders is almost certainly a result of a blown head gasket. The presence of coolant in the combustion chambers or in the engine crankcase will confirm this assumption.
If the compression in one of the cylinders is approximately 20% lower than in the others, and the idle speed is also unstable, then the cause may be excessive wear of the camshaft cam.
After completing the check, reinstall the spark plugs (refer to Chapter Routine care and maintenance, put the high-voltage wire tips on them, connect the connector to the ignition coil. Install the plastic engine cover and the fuel pump fuse in place.
Compression pressure values for various engines are given in Specifications.
Gasoline engine
1. Turn off the ignition.
2. 1.6L Engine: Disconnect the ignition coil output stage connector. The output stage is in the same housing as the ignition coils on the engine.
3. Remove fuel pump fuse No.28.
4. 1.8L Engine: Disconnect the ignition coil and injector connectors.
5. Engines 2.0, 2.4 and 3.0 l: Remove the ignition coils. Disconnect the injector connector.
6. Remove all spark plugs, refer to Section Checking and replacing spark plugs. Checking the condition of high-voltage wires.
Caution: Do not lean over the engine when cranking. There is a risk of injury from ejected soot particles.
7. Crank the engine with the starter to remove carbon deposits and soot.
Caution: Place the transmission in neutral and apply the parking brake.
8. Screw the compression gauge into the spark plug hole according to the manufacturer's instructions.
9. Ask your assistant to press the gas pedal and hold it with his foot during the test.
10. Crank the engine about 8 revolutions until there are no more pressure surges on the compression gauge.
11. Check all cylinders in sequence and compare the obtained pressure values with the specified ones.
12. Install the spark plugs and tighten them with torque 30Nm, please refer to Section Checking and replacing spark plugs. Checking the condition of high-voltage wires.
13. Reconnect all disconnected connectors, if necessary, replace the fuse.
Warning: When disconnecting the connectors, fault data is recorded in the engine control unit. It is recommended to delete the recorded data at an AUDI service station as soon as possible.
Diesel engines
1. 1.9L Engine: Disconnect the central engine plug. To do this, turn the knurled screw to the left. Disconnect the spark plug connector.
2. 2.5L Engine: Disconnect the fuel injection pump connector.
3. Remove all glow plugs, refer to Section Removal and installation glow plugs.
4. Screw in a compression gauge with a flexible connection instead of the glow plugs.
Warning: The following are listed in Specifications the compression values correspond to the measurement with the VAG compression tester. If other measuring devices are used, deviations may occur. In this case, it will only be possible to measure the pressure spread in individual cylinders.
5. Crank the engine about 8 revolutions until there are no more pressure surges on the compression gauge.
6. Install the glow plugs in place and tighten them with torque 15Nm. Connect the wire.
7. Connect and secure all connectors.
Caution: When disconnecting the connectors, fault data is written to the engine control unit. It is recommended to read and delete the written data as soon as possible.
Leak testing
During this test, the speed of the compressed air pumped into the cylinders and the leak locations are determined. This test is an alternative to the compression test. Moreover, from many points of view, it is much more effective, since it is easier to visually identify the source of the leak than to understand the results of the compression measurement.
The equipment required to check for leaks is not generally available, so this test will have to be entrusted to service center specialists.
[This article was copied from the website «AudiManual.ru»]