Note: An assistant will be required to check the compression pressure. Removing the fuel pump relay or disconnecting electrical components may store a fault code in the ECM and should then be cleared (see Chapter 5).
Note: On 1.9 and 2.0 diesel engines, ceramic glow plugs are installed, which should not be removed to check the compression pressure due to their high fragility - a diagnostic tool is used to check the compression pressure.
1. Measuring the compression pressure allows you to get a general idea of the current condition of such internal engine components as the 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 whether it is sufficient to replace only the cylinder head gasket. The measurement is performed using a compression gauge.
2. Make sure the engine temperature is at least 30°C.
3. Make sure the battery is fully charged (minimum 12.5 V).
4. On gasoline models, interrupt power supply to ignition coils (disconnect the ignition coil wiring connectors). Then remove the fuel pump relay or fuse, let the engine run until all the fuel remaining in the lines is used up (to relieve pressure in the fuel supply system), then remove all spark plugs (see Section 14 Chapter 1).
5. On diesel engines 2.7 and 3.0, remove all glow plugs and disconnect the injector wiring connectors.
6. Screw the compression tester adapter into the spark plug or glow plug hole.
Note: When measuring compression on diesel engines, make sure the compression tester has a sufficient measurement range.
7. Fully depress the gas pedal and, turning the engine with the starter, read and record the compression gauge readings when they stabilize (will stop growing).
Note: The measurement time should be as short as possible.
8. Repeat the procedures described in paragraphs 6 and 7 for the remaining engine cylinders.
9. Compare the measurement results with the requirements Specifications.
10. Compression in a healthy engine increases very quickly. A low value after the first cycle, increasing with subsequent cycles, indicates wear of the piston rings. A low value after the first cycle, not increasing after the following ones, indicates leaks through the valves, or a broken cylinder 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.
11. 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 or even lower, then to find out the reason, pour a teaspoon of engine oil into the cylinder through the spark plug hole and repeat the measurement.
12. If adding oil temporarily improved 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 reason is leaky valves or a broken cylinder head gasket.
13. 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.
14. If the compression in one of the cylinders differs from the others by more than 1 atm, and the idle speed is unstable, then the reason may be excessive wear of the camshaft cams.
15. After the test, disconnect the compression tester with the attachment, install the removed components and connect the wiring. Delete the fault codes from the ECM memory.