2. The engine must be fully warmed up to normal operating temperature, the battery must be fully charged and all spark plugs removed. An assistant is needed to carry out the operation.
3. Turn off the ignition system, to do this, disconnect the high-voltage wire of the ignition coil from the distributor cap and ground it on the cylinder block. On models with fuel injection, disconnect the fuel pump relay so that no fuel is injected when cranking the crankshaft (on carbureted models, the check valve connector can be disconnected).
4. Connect a compression gauge to the spark plug hole of the 1st cylinder (compression gauge with threaded tip is preferred).
5. Have an assistant open the throttle fully and engage the engine starter. After one or two revolutions of the engine, the pressure will rise to a maximum and then stabilize. Record the largest observed value.
6. Repeat the test on the remaining cylinders, each time recording the resulting compression value (see fig. 2.6).
Pic. 2.6. Compression gauge installed in spark plug hole
7. The pressure in all cylinders must be very close, a difference of more than 1.5 bar between the cylinders indicates a malfunction. It should be noted that the pressure in a serviceable engine increases rapidly, 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 leaky valve seal or a breakdown of the cylinder head gasket (Could also be a cracked head). Deposits on the valve surfaces can also cause low compression.
8. Recommended by Mercedes are given in the section Technical data.
9. If there is low pressure in any cylinder, do the following experiment to clarify the cause. Sip a teaspoon of clean oil into the spark plug hole and repeat the compression test.
10. If adding oil temporarily increases the compression pressure, it 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 blown head gasket.
11. Low pressure in two adjacent cylinders may indicate a breakdown of the gasket between them. The presence of coolant in the oil will confirm this assumption.
12. If the pressure in one of the cylinders is about 20% lower than in the others, and the engine is unstable at idle, then the cause may be wear on the camshaft cam.
13. If the compression pressure is unusually high, this may indicate that the combustion chambers are covered with a thick layer of soot. If this is the reason, it is necessary to remove the cylinder head and clean the chambers from carbon deposits.
14. After completing the check, screw the candles back into place and reconnect the ignition system and the fuel pump relay.