Compression Test — Description and Interpretation (Opel Astra G)

1. If the engine runs poorly or intermittently, check the ignition system and fuel system. If the reasons for poor engine operation cannot be found, perform a compression test. Regularly performing this test will also allow you to prevent malfunctions in advance.

2. To perform the check, the engine must be warm, the battery must be charged, and the spark plugs must be unscrewed.

3. Disable the ignition system by disconnecting the ignition coil high-tension wire from the distributor cap. Ground the wire to the cylinder block.

4. Connect a compression tester to the spark plug hole of cylinder No.1.

5. Open the throttle valve fully and turn the engine crankshaft with the starter. After one or two revolutions, the compression pressure should increase to the maximum mark and then stabilize. Write down the result.

6. Perform a similar check with the remaining cylinders and record the readings obtained.

7. The pressure in all cylinders should be the same. A difference of more than 2 bar between any two cylinders indicates a malfunction. In a healthy engine, the pressure should build up quickly. Low first-stroke pressure that gradually increases with subsequent strokes indicates worn piston rings. Low first-stroke pressure that does not build up with subsequent strokes indicates a leak in the valves or a blown head gasket. Deposits on the bottom of the valves can also cause low compression.

8. If you have any doubts during the inspection, contact specialists.

9. If the pressure in any cylinder is too low, perform the following test to determine the cause. Pour a teaspoon of clean motor oil into the cylinder through the spark plug hole and repeat the pressure test.



10 If the compression pressure increases after adding oil, this means that the piston or cylinder is worn. If the pressure does not increase, then the cause is worn or burnt valves or a broken cylinder head gasket.

11. Low pressure in two adjacent cylinders almost always means that the head gasket between them is blown.

12. If one cylinder has approximately 20% less pressure than the others and the engine idles poorly, this means that a worn camshaft lobe may be the cause.

13. If the pressure is very high, the combustion chambers are probably covered with carbon deposits.

14. After completing the check, screw the spark plugs back into place and connect the ignition system.