Note: An assistant will be required to check the compression pressure. When starting the engine with the fuses removed or the wiring disconnected, the corresponding DTC may be stored in the ECM memory, which should then be cleared (see chapter 5). Different design options for compression gauges and fluctuations in starter speed usually only allow you to compare compression in all cylinders.
Note: Checking engine compression pressure "VMR" should be done with the diagnostic equipment VAS5051A connected to the diagnostic tool.
1. Measurement of compression pressure provides an overview of the current condition of engine components such as cylinder head gasket, valve train components, pistons and piston rings. Analysis of the test results allows you to determine whether the engine needs a major overhaul, or it is enough to replace only the cylinder head gasket. The measurement is made using a compression gauge.
2. Warm up the engine to a temperature of at least 30°C and turn it off.
3. Make sure that the battery is fully charged and that the valve clearances are adjusted correctly.
Only for petrol models
4. Remove spark plugs (see Section 14 Chapter 1).
5. Remove the relay fuse ''J271' from the mounting block in the engine compartment" ECM supply.
6. Screw in the VAG1763 compression gauge instead of one spark plug.
Only for diesel models
7. Prevent fuel supply, turn out the corresponding glow plug (see chapter 5), screw the VAG1381 / 12 adapter in its place and connect the VAG1763 compression tester to it (see illustration 2.7a). To prevent fuel supply on 1.9L engines, disconnect the central connector of the unit injectors and the extreme connector of the glow plugs, and on CFFB / CLJA / CFGB engines, disconnect the connector of the fuel pressure control valve (see illustration 2.7b).
2.7а Compressometer with adapter connected to 1.9 l engine
2.7b Fuel pressure control valve connector for CFFB/CLJA/CFGB engines
All models
8. Fully depress the gas pedal and, turning the engine with the starter, read and record the compression gauge readings when they stabilize (stop increasing).
Note: The measurement time should be as short as possible.
9. Check the compression pressure in the remaining engine cylinders and compare the measurement results with the requirements Specifications.
10. Compression in a serviceable engine grows very quickly. A low reading after the first cycle, increasing with successive cycles, indicates worn piston rings. A low value after the first cycle, which does not increase after the next, indicates the presence of leaks through the valves, or a blown cylinder head gasket (Could also be a cracked head). The presence of soot on the valve plates can lead to a decrease in compression.
11. The results obtained when measuring compression should be approximately the same for all cylinders.
If the pressure in any cylinder is at the level of the minimum allowable (see specs) and even lower, then to find out the reason, pour a teaspoon of impellent oil into the cylinder through the candle hole and repeat the measurement.
12. If the addition of oil temporarily improves compression, the reason for the decrease is most likely piston, ring or cylinder wear. If there is no increase in compression, then it can be assumed that the cause is a loose fit of the valves or a broken cylinder head gasket.
13. 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 crankcase will confirm this assumption.
14. If the compression in one of the cylinders differs from the rest by more than 1 bar, besides, the idle speed is unstable, then the reason may be excessive wear of the camshaft cam.
15. After checking, disconnect the compression gauge with adapter (if used), install the removed components and connect the electrical wiring. Tighten glow plugs for 1.9 l engines with a force of 15 Nm.
16. Finally, use the diagnostic tool to clear the errors from the ECM memory.