Note: An assistant will be required to check the compression pressure. If the engine is started with the wiring disconnected, a corresponding fault code may be stored in the ECM memory, which should then be cleared (see Chapter 5). Various designs of compression gauges and fluctuations in the starter speed usually only allow us to compare the compression in all cylinders.
1. Measuring compression pressure allows you to get a general idea of the current condition of engine components such as the cylinder head gasket, valve train components, pistons and piston rings. Analysis of the test results allows us to determine whether the engine requires a major overhaul. or it is enough to replace only the cylinder head gasket. The measurement is performed using a compression gauge.
2. Warm up the engine to a temperature of approximately 80°C and turn it off.
3. Make sure the battery is fully charged (the voltage must be at least 12.5 volts) and in the correct adjustment of valve clearances.
4. Remove the upper engine cover (see Section 19 of Chapter 1).
5. Disconnect the glow plug wiring connectors (see Chapter 5) and the electrical wiring connector of the "N276" fuel pressure control valve on the fuel distribution line of the right cylinder head (see illustrations). Run the engine briefly to relieve fuel pressure in the line.
2.5a. Fuel pressure control valve connector "N276" (3.0 TDI engines of the 1st generation and SCR, as well as the 4.2 TDI engine with the BTR code).
2.5b. Fuel pressure control valve connector "N276" (3.0 TDI engines 2nd generation).
2.5 sec. Fuel pressure control valve connector "N276" (4.2 TDI engines with CCFA/CCFC codes).
6. Remove all glow plugs (see illustration), as described in Chapter 5, screw the VAG1763/8 adapter into the spark plug hole of the cylinder being tested and connect the VAG1763 compression tester to it.
2.6. Removing the glow plug (using the example of 3.0 TDI engines of the 1st generation and SCR; on 2nd generation engines - similarly).
7. Fully depress the gas pedal and, cranking the engine with the starter, read and record the compression gauge readings when they stabilize (will stop increasing).
Note: The measurement time should be as short as possible.
8. Check the compression pressure in the remaining engine cylinders and compare the measurement results with the requirements Specifications.
9. Compression in a healthy engine increases very quickly. A low value shown after the first cycle, increasing with subsequent cycles, indicates worn piston rings. A low reading after the first cycle that does not increase after the following cycles indicates leaks through the valves or a blown cylinder head gasket (a crack in the head could also be the cause). The presence of carbon deposits on the valve plates can lead to a decrease in compression.
10. The results obtained when measuring compression should be approximately the same for all cylinders. If the pressure in any cylinder is at the minimum permissible level (see Specifications) and even lower, then to find out the cause, pour a teaspoon of engine oil into the cylinder through the spark plug hole and repeat the measurement.
11. If adding oil temporarily improves compression, the cause of 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 a loose valve fit or a broken cylinder head gasket.
12. Low compression in two adjacent cylinders is almost certainly the result of a blown head gasket. The presence of coolant in the combustion chambers or in the engine crankcase will confirm this assumption.
13. If the compression in one of the cylinders differs from the others by more than 1 bar, and the idle speed is also unstable, then the cause may be excessive wear of the camshaft cam.
14. After completing the check, disconnect the compression tester from the adapter (if used), install the removed components and connect the electrical wiring.
15. Finally, use a diagnostic tool to clear the errors from the ECM memory.