Compression is checked with a special device - a compression meter, which can now be freely purchased in large 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 threaded nipple for screwing in the spark plug. When checking compression, these compression testers are simply pressed firmly against the spark plug hole.
WARNING: Important conditions for correct readings when checking compression are the serviceability of the starter and its electrical circuits, as well as a fully charged battery.
1. Start the engine and warm it up to operating temperature.
2. Reduce the pressure in the engine fuel system (see "Reducing the pressure in the engine fuel system"). Do not install the fuel pump circuit fuse in the central mounting block.
3. Remove the spark plug wire ends and unscrew all the spark plugs (see "Replacing and servicing spark plugs").
4. Disconnect the low voltage wire harness connector from the ignition coil, thereby disabling the ignition system.
WARNING: Cranking the engine with the starter motor with the high-voltage wires disconnected and the ignition coil not disconnected may cause a breakdown in its high-voltage circuit.
5. Screw the compression gauge into the spark plug hole of the cylinder being tested.
6. Press the accelerator pedal all the way down so that when the crankshaft is turned by the starter, the throttle valve is fully open in the engine cylinder purging mode.
7. 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 a correct compression tester reading, the crankshaft must rotate at 180–200 min⁻¹ or higher, but not more than 350 min⁻¹.
8. Having recorded the compression meter readings…
9. …set its arrow to zero by pressing the air release valve.
NOTE: For compression gauges of a different design, readings may be reset in other ways (in accordance with the instructions for the device).
10. Repeat steps 5–9 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.
11. 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, this means that the valve plates are not tightly seated on their seats or the cylinder head gasket is damaged.
USEFUL TIP: 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.