An assistant will be required 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. Warm up the engine to normal operating temperature (about 80 degrees.) and muffle it.
3. Make sure the battery is fully charged.
4. On petrol models disconnect the ignition coil wiring connectors.
5. On the engine M52 remove the ECM relay; on other gasoline engines remove the fuel pump relay or fuse, let the engine run until all the fuel remaining in the lines is used up, then remove the spark plugs.
6. On gasoline engines, remove the spark plugs, and on diesel - glow plugs.
7. On diesel models, depending on the engine type, do the following:
- M51: Remove the ECM main relay.
- M57: Disconnect the injector wiring connectors and open all suction channels by removing any existing covers, gaskets, etc.
- M67: Disconnect the camshaft position (CMP) sensor wiring connector.
8. Screw the compression tester adapter into the spark plug or glow plug hole.
To measure the compression of diesel engines, you should make sure that the compression tester has a sufficient measurement range.
9. Fully depress the gas pedal and, turning the engine with the starter, read and record the compression gauge readings when they stabilize.
The measurement time should be as short as possible.
10. Repeat the procedures described in paragraphs 8 and 9 for the remaining engine cylinders.
11. Compare the measurement results with the requirements Specifications.
12. 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.
13. 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.
14. 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.
15. 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.
16. 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 cam.
17. After completing the check, disconnect the compression gauge with the attachment, install the removed components and connect the electrical wiring.