Checking the compression pressure in the cylinders (Opel Astra H)

1. The results of the cylinder compression pressure test allow you to determine the general condition of the components of the upper part of the engine (pistons, piston rings, valves, cylinder head gaskets, etc.) and qualitatively assess the degree of their wear. What is especially important is that the analysis of the information obtained as a result of the check allows us to narrow the range of possible causes of the compression drop in the cylinders, quite accurately linking the failure to malfunctions of very specific components, be it piston rings, valves, their seats or head gaskets.

Note: The engine must be warmed up to normal operating temperature (at least 80°C) and the battery fully charged.

2. Start by cleaning the spark plug niches (blow them out with compressed air, a regular bicycle pump is a great source of air in this case). The main objective is to prevent debris from entering the cylinders during measurements.

3. Depending on the model, it is first necessary to remove the engine cover and disconnect all elements of the engine air supply system and engine crankcase ventilation that may interfere with the removal of the ignition module or block access to the glow plugs of a diesel engine.

4. On a gasoline engine, remove the ignition module and spark plugs (see Chapter 1, Section 22).

Attention: Work is carried out on a hot engine - do not touch the heated parts of the engine with your hands, use special handles/bolts to remove the module!

On diesel engines, disconnect the supply wiring from the glow plugs and remove the glow plugs (see Chapter 4, Section 11).



5. Using the OBD on-board diagnostic system connector (see Chapter 5) and the TECH-2 device (program "Compression test") open the engine throttle valve.

6. Install the compression gauge into the spark plug hole of the first cylinder (see accompanying illustrations), using the appropriate adapter for the given engine.

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Note: When installing the compression gauge, do not forget to check for the presence of a rubber sealing gasket on the adapter.

The compression meter measuring device must be set to the appropriate rank for taking measurements - for petrol engines 1750 kPa (17.5 bar). In diesel engines, the compression ratio is much higher than in petrol engines, so a special compression meter with a measurement range of 10 to 40 bar is required to check the compression pressure.



7. Crank the engine with the starter for about 4 seconds (use the help of an assistant) in this case, the engine crankshaft rotation speed must be at least 300 rpm for gasoline engines and at least 150 rpm for diesel engines. Read and record the meter reading. Repeat the operation for the remaining cylinders. Compare the readings with the standard (see Specifications) — the pressure in any of the cylinders should not be less than 75% of the standard, while the pressure difference in different cylinders should not exceed 100 kPa (1 bar).

8. Equally low measurement results in all cylinders usually indicate wear of the compression rings in all cylinders and the need to replace them. Excessive pressure drop in one or more cylinders indicates the occurrence of local faults. When low compression pressure is detected in the engine cylinders, a more thorough check is carried out to accurately determine the cause - for example, a leak test (see below). Such a check is usually carried out in a car service workshop.

9. There are several practical ways to more accurately determine the cause of the malfunction. For example, if you add 2-3 teaspoons of engine oil to a cylinder in which excessively low pressure is determined, then an increase in pressure upon repeated measurement indicates wear of the piston rings. If the compression does not increase, the cause of its decrease is valve leaks or a leak in the head gasket.

10. If the compression pressure is equally low in two adjacent cylinders, then with a high degree of probability we can speak about a violation of the integrity of the head gasket within its bridge between these cylinders. The presence of coolant in the engine oil will confirm this assumption.



11. After completing the check, disconnect the compression gauge and reinstall the removed components.

Checking the block for leaks

15. 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, but it requires a slightly larger volume of preparatory work and the use of a special device for measuring leaks, the handling of which requires certain skills, in addition, for the convenience of visual inspection, it is preferable to use a special lift.

16. The equipment required to check for leaks is not generally available, so this test will have to be entrusted to service center specialists.