1. Warm up the engine to operating temperature. The battery must be fully charged. Unscrew the spark plugs.
2. Disconnect the electrical wires from the ignition distributor. On fuel-injected engines, remove the fuel pump relay, and on carburetor models, disconnect the electrical connector from the fuel shut-off valve.
3. Install a compression gauge into the spark plug hole of the first cylinder.
4. The assistant should press the accelerator pedal all the way down and turn the engine over with the starter. After turning the crankshaft one or two revolutions, the compression pressure should increase to the maximum and then stabilize. Record the maximum result obtained.
5. Check the remaining cylinders in the same way.
[Material created based on the website: MercedesMan.ru]
6. The compression pressure in all cylinders should not differ by more than 1.5 bar. Note that the compression value should increase rapidly on a serviceable engine. Low compression pressure on the first revolution of the crankshaft, followed by a gradual increase in pressure on subsequent revolutions of the crankshaft indicates wear of the piston rings.
7. A low pressure reading on the first revolution of the crankshaft, which increases slightly thereafter, indicates a loose valve fit, a broken cylinder head gasket, or a crack in the cylinder head. Worn valve stem ends can also cause low compression pressure.
8. If the pressure in one cylinder is low, then perform the following test. Pour some engine oil into the cylinder through the spark plug hole and check the compression pressure.
9. If pouring oil into the cylinder increases the compression pressure, this indicates that the cylinder or piston rings are worn.
10. Low pressure in two adjacent cylinders indicates a burnt-out gasket between the cylinders.
11. If the pressure in one cylinder is 20% less than in the others, and the engine runs rough at idle, this indicates a worn camshaft cam.
12. If the compression pressure is too high, this indicates that the combustion chamber is covered with a thick layer of carbon deposits.
13. After this check, screw in the spark plugs, connect the high-tension wires and install the fuel pump relay.