2. The engine must be fully warmed up to normal operating temperature, the battery must be fully charged, and all spark plugs must be removed. An assistant will be needed to perform the operation.
3. Disable the ignition system by disconnecting the ignition coil high-voltage wire from the distributor cap and grounding it to the cylinder block. On fuel-injected models, disconnect the fuel pump relay to prevent fuel from being injected when the crankshaft is cranked (on carbureted models, the shut-off valve connector can be disconnected).
4. Connect the compression gauge to the spark plug hole of the 1st cylinder (a compression gauge with a threaded tip is preferred).
5. Have your assistant open the throttle fully and engage the engine starter. After one or two engine revolutions, the pressure will rise to its maximum and then stabilize. Record the highest value observed.
6. Repeat the test on the remaining cylinders, recording the resulting compression value each time (see fig. 2.6).
Fig. 2.6. Compression gauge installed in the spark plug hole
7. The pressure in all cylinders should be very close, a difference of more than 1.5 bar between cylinders indicates a malfunction. It should be noted that the pressure in a serviceable engine increases quickly, low pressure in the first cycle and its gradual increase in subsequent cycles means wear of the piston tracks. Low pressure in the first cycle, which does not increase in subsequent cycles, indicates a loose valve fit or a breakdown of the cylinder head gasket (the cause could also be a crack in the head). Deposits on the valve working surfaces can also cause low compression.
Information copied from the website [MercedesMAN.ru]
8. Recommended by Mercedes are listed in the section Technical data.
9. If low pressure is observed in any cylinder, perform the following experiment to determine the cause. Pour a teaspoon of clean oil into the spark plug hole and repeat the compression test.
10. If adding oil temporarily increases the compression pressure, this means that cylinder or piston wear is causing gas leakage and pressure loss. If there is no improvement, it should be assumed that there is a leak or burnt valve, as well as a broken head gasket.
11. Low pressure in two adjacent cylinders may indicate a gasket failure between them. The presence of coolant in the oil will confirm this assumption.
12. If the pressure in one of the cylinders is approximately 20% lower than in the others, and the engine runs unsteadily at idle, then the cause may be wear of the camshaft cam.
13. If the compression pressure is unusually high, it may mean that the combustion chambers are covered with a thick layer of carbon deposits. If this is the case, it is necessary to remove the cylinder head and clean the chambers from carbon deposits.
14. After completing the check, screw the spark plugs back in place and reconnect the ignition system and fuel pump relay.