Note. An assistant will be required to perform the compression test procedure.
1. Compression gives an idea of the state of the engine. Test results show if valves and pistons are OK (piston rings), or they are worn out. In addition, the test results make it possible to decide whether to replace the engine, whether it needs a complete overhaul, or whether it is enough to replace the cylinder head gasket. To check, a compression tester is required, which for gasoline engines is available in specialized stores at an affordable price.
Note. For diesel engines, a compression tester with a measuring range of up to approx. 40 bar is required.
2. Warm up the engine to normal operating temperature (about 80°С), make sure the battery is fully charged.
Petrol models
1. Shut off the engine and, as supplied, remove the power unit cover, air cleaner assembly and, if equipped, the resonator chamber with connecting sleeve (see chapter Servicing Air Inlet Components).
2. Turn out spark plugs and turn the engine over with a starter.
3. Fill the nozzle of the compression gauge into the spark plug hole of the first cylinder.
4. Squeeze out the gas pedal and, turning the engine with a starter, read the meter readings (need help from an assistant), - record the measurement result.
5. Proceeding in a similar manner, alternately measure the compression pressure in the remaining engine cylinders.
6. Stop the engine, disconnect the compression tester, screw in the spark plugs and reinstall all the components that were removed to provide access.
Diesel models
Stop the engine and remove the cover of the air distribution sleeve of the pressurization system path and the cylinder head cover trim.
Checking Compression Pressure on Diesel Models Equipped with 612 Series Engine
1 - Cover of the air distribution sleeve of the pressurization system tract; 2 - Overlay of a cover of a head of cylinders; 3 - Compressometer; 4 - Connecting hose; 5 - Threaded nozzle; R9 - Glow Plug
Checking Compression Pressure on Diesel Models Equipped with 628 Series Engine
1 - Cover of the air distribution sleeve of the pressurization system tract; 2 - Overlay of a cover of a head of cylinders; 3 - Compressometer; 4 - Connecting hose; 5 - Threaded nozzle; R9 - Glow plug; Y74 - Pressure control valve
M612
1. Remove the glow plugs.
2. Using a threaded nozzle with a flexible connecting hose, connect the compression tester to the spark plug hole of the first cylinder.
3. Then proceed in the same manner as described above for gasoline engines.
M628
Checking Compression Pressure on Diesel Models Equipped with 628 Series Engine
3 - Compressometer; 5 - Threaded nozzle; 6 - Assembly of the air cleaner; 7 - Pin connector; R9 - Glow plug; Y74 - Pressure control valve
1. Remove the air cleaner assembly.
2. Disconnect the air distribution pipe of the intake air path.
3. Disconnect the pressure control valve connector
4. Remove the glow plugs.
5. Proceed further in accordance with the instructions given for gasoline engines.
All models
1. Compare the measurement results with the requirements of the Specifications.
2. The maximum allowable pressure difference in the individual cylinders can be 1.5 bar (for diesel engines 3.0 bar).
3. Compression in a serviceable engine grows very quickly. A low reading after the first cycle, increasing with successive cycles, indicates worn piston rings. A low value after the first cycle, not increasing after the next, signals that either the valves are leaking or the head gasket is blown (Could also be a cracked head). The presence of soot on the valve plates can lead to a decrease in compression.
4. Please note that due to different starter rotation speeds on different models, the results obtained as a result of measurements may vary. The results obtained when measuring compression should be approximately the same for all cylinders.
5. If the pressure in any cylinder is at or below the minimum allowable pressure, follow these steps to determine the cause. Pour a teaspoon of engine oil into the cylinder through the spark plug hole and repeat the compression measurement.
6. If the addition of oil temporarily improves compression, the reason for the decrease is most likely piston, ring or cylinder wear. If there is no increase in compression, then it can be assumed that the cause is valve leaks or a broken head gasket.
7. Low compression in two adjacent cylinders is almost certainly the result of a blown head gasket. The presence of coolant in the combustion chambers or in the crankcase will confirm this assumption.
8. If the compression in one of the cylinders is about 20% lower than in the others, besides, the idle speed is unstable, then the cause may be excessive wear of the camshaft cam.
9. After completing the check, reinstall the spark plugs (contact the head Ongoing care and maintenance), put on them tips of high-voltage wires, connect the connector to the ignition coil. Reinstall the plastic engine cover and fuel pump fuse.
Checking cylinders for leaks
1. During this test, the rate of exit from the cylinders of the compressed air pumped into them and the leaks directly are determined. This test is an alternative to the compression test. Moreover, from many points of view, it is much more effective, since visually identifying the source of a leak is easier than comprehending the results of a compression measurement.
2. The equipment required to test for leaks is not publicly available, so this test will have to be entrusted to the service center specialists.