Checking compression pressure, assessing the condition of the cylinders (Mercedes-Benz G-Class W463)

Checking the compression pressure

Note: An assistant will be required to perform the compression pressure test procedure.

Measuring the compression pressure allows you to get a general idea of the current condition of such internal engine components as cylinder head gaskets, valve train components, pistons and piston rings. Analysis of the test results allows you to determine whether the engine needs a major overhaul or whether it is enough to simply replace the head gasket. The measurement is performed using a compression tester.

Note: For diesel engines, a compression tester with a measurement range of approximately 40 atm is required.

Warm up the engine to normal operating temperature (around 80°C), make sure the battery is fully charged.

Petrol models

To measure the compression pressure, use a meter equipped with a remote starter switch (shown on the example of model 202).



1. Turn off the engine and remove the air cleaner assembly as specified in the package (see Chapter Fuel and exhaust systems).



2. Remove the spark plugs one by one (see Chapter Engine electrical systems).

3. Remove the key from the electronic ignition switch (EIS).

4. Prepare a compression tester equipped with a remote starter switch

5. Remove the cover (1) of the control module mounting block



6. Connect the clamp (A) of the measuring device to the terminal X12/3.



7. Disconnect the X35/63 connector and connect the adapter cable (2) equipped with a 6-pin plug to it, - connect the other end of the cable to the compression tester cable

Caution! When measuring compression, do not turn the starter from the ignition switch!

8. Press the measuring nozzle firmly against the bottom of the spark plug niche of the first cylinder.



9. Open the throttle valve completely and, turning the engine with the starter activated by the remote switch button of the compression gauge, read the readings of the latter. Write down the measurement result.

Note: To remove carbon deposits from the cylinder walls, the engine should be cranked for about 5 seconds.

10. Proceeding in a similar manner, measure the compression pressure in the remaining engine cylinders one by one.

11. Turn off the engine, disconnect the compression tester, screw in the spark plugs and install the air cleaner.

Diesel models

Checking Compression Pressure on Diesel Models Equipped with a 612 Series Engine





Checking Compression Pressure on Diesel Models Equipped with a 628 Series Engine



1. Stop the engine and remove the air distributor cover and the cylinder head cover/air cleaner assembly (see Chapter Fuel and exhaust systems). Turn the ignition key to position 0.

M612

To measure the compression pressure, use a meter equipped with a remote starter switch (shown on the example of model 202)



1. Remove the glow plugs (see Chapter Engine electrical systems).

2. Prepare a compression tester equipped with a remote starter switch.



3. Remove the cover (1) of the external starting block (2) and connect the measuring clamp to the terminal X4/30



4. Remove the cover (arrow) on the right under the instrument panel



5. Remove the starter relay (3) from the mounting block, the installation point of which is marked with the letter S.



6. To the block (4) of the relay (letter S on the mounting block board) connect the compression tester adapter cable

Attention! When measuring compression, do not turn the starter from the ignition switch!

7. Using a threaded nozzle with a flexible connecting hose, connect the compression tester to the spark plug hole of the first cylinder (see illustration Checking compression pressure on diesel models equipped with a 612 series engine).

8. Then proceed in accordance with the instructions given for petrol engines - there is no need to press the gas pedal/open the throttle valve.

Note: The engine must be turned over at least eight times to perform the measurement.

M628

1. Disconnect the pressure regulator valve connector (see illustration Checking Compression Pressure on Diesel Models Equipped with a 628 Series Engine) and remove the glow plugs (see Chapter Engine electrical systems).

2. Prepare a compression tester equipped with a remote starter switch (see illustration Checking Compression Pressure on Diesel Models Equipped with a 628 Series Engine) and, acting further in accordance with the instructions (using the example of models 220.028/128, equipped with engines 629.960), connect it to the terminals of the corresponding fuse in the right front mounting block.



3. Remove the cover of the right front fuse and relay mounting block (K40/7) and remove the maxi fuse (60A) for the engine cooling fan from its socket, then disconnect the white connector (1)



4. Connect the compression tester adapter leads to the base terminal (30) of the fuse socket (33) and terminal (50) of the white connector (1)

Attention! When measuring compression, do not turn the starter from the ignition switch!

5. Using a threaded nozzle with a flexible connecting hose, connect the compression tester to the spark plug hole of the first cylinder (see illustration Checking Compression Pressure on Diesel Models Equipped with a 628 Series Engine).

6. Then proceed in accordance with the instructions given for petrol engines - there is no need to press the gas pedal/open the throttle valve.

Note: The engine must be turned over at least eight times to perform the measurement.

7. Alternatively, the compression pressure can be measured using the STAR DIAGNOSIS diagnostic reader - follow the instructions supplied with the device.

All models

1. Compare the measurement results with the requirements Specifications.

2. The maximum permissible pressure difference in individual cylinders can be 1.5 atm (for gasoline engines) and 3.0 atm (for diesel engines).

3. 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 leaky 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.

4. It should be noted that due to the different starter speeds on different models, the results obtained from measurements may differ. The results obtained from measuring compression should be approximately the same for all cylinders.

5. 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.

6. 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 cylinder head gasket.

7. 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.

8. 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.

9. After completing the check, reinstall the spark plugs, reconnect the wiring to them, and on gasoline engines, restore the original connection of the ignition coil(s). Reinstall the plastic engine cover and the fuel pump fuse.

Checking cylinders for leaks

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.

All models

1. Warm up the engine to normal operating temperature (80°C), then turn it off.

Petrol models

Checking petrol engine cylinders for leaks



1. Carefully turn the cap (2) of the expansion tank (1) of the cooling system, equipped with a built-in latch, to the first stop, wait until the pressure is released, then unscrew the cap completely.

2. Remove the spark plugs (19).

3. Turning the engine by the central bolt of the crankshaft in the normal direction, bring the piston of the cylinder to be tested to the TDC position of the end of the compression stroke, then connect a special tester (04) to the corresponding spark plug hole, having previously calibrated it - act in strict accordance with the instructions attached to the device.

4. Remove the engine oil filler cap (3).

5. Supply compressed air to the cylinder and use the meter readings to estimate the rate of pressure release. Record the result.

Note: If the crankshaft starts to turn during the check, block it with a special device (see Sections Checking and adjusting the basic position of the camshafts and Checking the basic position of the balance shaft (only engines M112 and 628)).

6. Repeat the test for the remaining cylinders, proceeding in the firing order (see Chapter Engine electrical systems).

7. Disconnect the gauge and screw it back into place with the spark plugs.

Diesel models

Checking diesel engine cylinders for leaks



1. Turn the cap (1) of the expansion tank (2) of the cooling system counterclockwise by half a turn, wait until the pressure is released, then remove the cap completely.

2. Remove the engine trim panels (M612)/air cleaner assembly (M628).

3. Remove the glow plugs (R9).

4. On models equipped with a 628 series engine, remove the charge air cooler (see Chapter Fuel and exhaust systems).

5. On models equipped with a 612 series engine, remove the cooling fan viscous coupling (see Chapter Cooling, heating, ventilation and air conditioning systems).

6. Turning the engine by the central bolt of the crankshaft in the normal direction, bring the piston of the cylinder to be tested to the TDC position of the end of the compression stroke, then connect a special tester (04) to the corresponding spark plug hole, having previously calibrated it - act in strict accordance with the instructions attached to the device.

7. Then proceed in a manner similar to that described above for gasoline engines (see paragraphs 4 through 7 of Checking cylinders for leaks - petrol models).

All models

1. Compare the test results with the requirements Specifications. If necessary, carry out the necessary restorative repairs.

Checking cylinders with a test lamp

Petrol models

Checking petrol engine cylinders using a test lamp



1. Remove the spark plugs.

2. Turning the engine in the normal direction using the central bolt of the crankshaft, bring the piston of the cylinder to be checked to the BDC position.

3. Following the manufacturer's instructions strictly, connect the special test lamp (05), insert its flexible light guide into the corresponding spark plug hole and visually assess the condition of the cylinder walls, which should have a matte finish without any traces of honing notches.

4. Proceeding in a similar manner, inspect the walls of the remaining cylinders (in order of ignition).

5. If necessary, remove the engine, cylinder heads and carry out a more detailed direct inspection of the cylinder mirrors and the necessary restorative repairs.

Diesel models

1. Remove the ignition key from the lock.

2. Remove the cylinder head trim panels (M612)/air cleaner assembly (M628).

3. Remove the glow plugs (R9).

4. Remove the injectors (Y76) (see Chapter Fuel and exhaust systems).

5. On models equipped with a 628 series engine, remove the charge air cooler (see Chapter Fuel and exhaust systems).

6. On models equipped with a 612 series engine, remove the cooling fan viscous coupling (see Chapter Cooling, heating, ventilation and air conditioning systems).

7. Turning the engine by the central bolt of the crankshaft in the normal direction, bring the piston of the cylinder to be checked to the BDC position.

8. Then proceed in a manner similar to that described above for gasoline engines (see paragraphs 3 through 5).