Attention! For diesel engines, a compression tester with a measurement range of approximately 40 bar is required.
2. The engine must be warmed up to operating temperature, the battery must be fully charged. You will also need the help of an assistant.
3. Remove the fuel pump fuse from the block located in the engine compartment (refer to Chapter On-board electrical equipment). Start the engine and wait until it stalls. If the engine does not start, crank the starter for about 10 seconds.
4. To access the spark plugs, remove the oil filler cap and remove the plastic engine cover. Replace the cap.
5. Disconnect the ignition system by disconnecting the connector from the ignition coil. Remove all spark plugs (refer to Section Checking and replacing spark plugs. Checking the condition of high-voltage wires).
6. Install the compression tester into the spark plug hole of the first cylinder. It is advisable to use a compression tester whose tip is screwed into the spark plug hole.
7. Ask an assistant to keep the accelerator pedal fully depressed and turn on the starter. After several cycles, the compression in the cylinder will reach its maximum value and will not change any more. Write down the result.
8. The maximum permissible pressure difference in individual cylinders can be 3.0 bar (for diesel engines 5.0 bar).
9. 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, signals that either the valves are leaking or the head gasket is blown (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.
10. It should be noted that due to different starter speeds on different models, the results obtained from measurements may differ. The results obtained from compression measurements should be approximately the same for all cylinders.
11. If the pressure in any cylinder is at the minimum permissible level or even lower, then to find out the reason, do the following: Pour a teaspoon of engine oil into the cylinder through the spark plug hole and repeat the compression 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 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 is approximately 20% lower than in the others, and the idle speed is unstable, then the reason may be excessive wear of the camshaft cam.
15. After completing the check, reinstall the spark plugs (refer to Chapter Routine maintenance), put the high-voltage wire tips on them, connect the connector to the ignition coil. Install the plastic engine cover and the fuel pump fuse in place.
16. The compression pressure values for different engines are given in Specifications.
Leak testing
1. During this test, the speed of the compressed air pumped into the cylinders and the leak locations are determined. This test is an alternative to the compression test. Moreover, from many points of view, it is much more effective, since it is easier to visually identify the source of the leak than to understand the results of the compression measurement.
Information copied from the website [mercedesman]
2. The equipment required to check for leaks is not generally available, so this test will have to be entrusted to service center specialists.