An assistant will be required to check the compression pressure.
1. Measurement of compression pressure provides an overview of the current condition of internal 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 normal operating temperature (about 80 deg.) and mute it.
3. Make sure the battery is fully charged.
4. On petrol models disconnect sockets of electroconducting of coils of ignition.
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 the remaining fuel in the lines is completely used up, then remove the spark plugs.
6. On gasoline engines, turn out the spark plugs, and on diesel - glow plugs.
7. On diesel models, depending on the type of engine, do the following:
- M51: Remove the ECM main relay.
- M57: Disconnect the electrical connectors of the injectors and open all suction channels, to do this, remove the existing covers, gaskets, etc.
- M67: Disconnect the camshaft position sensor wiring connector (CMP).
8. Screw the adapter nozzle of the compression gauge into the hole of the spark plug or glow plug.
To measure the compression of diesel engines, make sure that the compression gauge has a sufficient measuring range.
9. Fully depress the gas pedal and, turning the engine with a starter, read and write down 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 measurement results with requirements Specifications.
12. 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 that does not increase after the next cycle indicates valve leaks 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.
13. 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 and even lower, then to find out the reason, pour a teaspoon of engine oil into the cylinder through the candle hole and repeat the measurement.
14. If the addition of oil has temporarily improved compression, the cause of 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 valve leaks or a broken head gasket.
15. 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.
16. If the compression in one of the cylinders differs from the rest by more than 1 atm, besides, the idle speed is unstable, then the reason may be excessive wear of the camshaft cam.
17. After the test, disconnect the compression tester with a nozzle, install the removed components and connect the electrical wiring.