Checking the compression in the cylinders (Opel Astra J)

Compression (compression stroke end pressure) in cylinders is the most important indicator for diagnosing the condition of the engine without disassembling it. Based on its average value and the difference in values in individual cylinders, it is possible to determine with sufficient accuracy the degree of general wear of the engine connecting rod and piston group parts, identify faults in this group and valve mechanism parts.

Compression is checked with a special device - a compression meter, which can now be freely purchased in auto parts stores.

Note: This is what the compression tester used to illustrate this book looks like. There are versions of compression testers that have a rubber tip instead of a spark plug instead of a threaded fitting for screwing in. When checking compression, such compression testers are simply pressed hard against the spark plug hole.

Attention! An important condition for the correctness of the readings when checking the compression is the serviceability of the starter and its electrical circuits, as well as the full charge of the battery.

1. Start the engine and warm it up to operating temperature.



2. Reduce the pressure in the fuel system (see Reducing pressure in the fuel system).

Note: After the pressure has dropped, do not replace the fuel pump fuse to disable the fuel pump.

3. Remove the ignition module and remove all spark plugs (see Replacement and maintenance of spark plugs).



4. Screw the compression gauge into the spark plug hole of the cylinder being tested.

5. Press the accelerator pedal all the way down to fully open the throttle valve.

6. Turn on the starter and turn the engine crankshaft until the pressure in the cylinder stops increasing. This corresponds to approximately four compression strokes.

Note: To obtain correct compression tester readings, the crankshaft must rotate at 180–200 rpm or higher, but not more than 350 rpm.

(Material copied from this website: OpelBook)

7. Having recorded the compression meter readings…



8. …set its arrow to zero by pressing the air release valve.

Note: The readings of compression meters of other designs can be reset in other ways in accordance with the instructions for the device.

9. Repeat steps 4–8 for the remaining cylinders. The pressure should be no lower than 1.0 MPa and should not differ by more than 0.1 MPa in different cylinders. Low compression in individual cylinders may result from loose valve seats, damaged cylinder head gaskets, broken or burnt piston rings. Low compression in all cylinders indicates worn piston rings.

10. To determine the causes of insufficient compression, pour about 20 ml of clean motor oil into the cylinder with low compression and measure the compression again. If the compression meter readings have increased, the piston rings are most likely faulty. If the compression remains unchanged, the valve plates do not fit tightly to their seats or the cylinder head gasket is damaged.

Note. The cause of insufficient compression can also be determined by feeding compressed air into the cylinder, in which the piston is preliminarily set to TDC of the compression stroke. To do this, remove the tip from the compression tester and attach the compressor hose to it. Insert the tip into the spark plug hole and feed air into the cylinder under a pressure of 0.2–0.3 MPa. To prevent the engine crankshaft from turning, engage the highest gear and brake the car with the parking brake. Air escaping (leaking) through the throttle assembly indicates a leak in the intake valve, and through the muffler - a leak in the exhaust valve. If the cylinder head gasket is damaged, air will escape through the neck of the expansion tank in the form of bubbles or into the adjacent cylinder, which is detected by a characteristic hissing sound.