Compression is checked with a special device - a compression gauge, which can now be freely purchased at large auto parts stores.
Note. This is what the compression gauge used to illustrate this book looks like. There are variants of compression meters that have a rubber tip instead of a threaded fitting for screwing in instead of a spark plug. Such compression gauges, when checking compression, are simply strongly pressed against the candle hole.
Attention! Important conditions for the correctness of the readings when checking compression are 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 supply system (see «Reduced fuel pressure in the engine power system»). After reducing the pressure, do not install the fuel pump fuse in place to turn off the fuel pump.
3. Remove ignition coils 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 to be checked.
5. Fully depress the accelerator pedal to fully open the throttle.
6. Turn on the starter and turn the engine crankshaft with it until the pressure in the cylinder stops increasing. This corresponds to about four compression strokes.
Note. To obtain correct compression gauge readings, the crankshaft must rotate at a speed of 180–200 min-1 or higher, but not more than 350 min-1.
7. Having recorded the readings of the compression gauge, set its arrow to zero by pressing the air release valve.
Note. For compression meters of a different design, readings can be reset in other ways (in accordance with the instructions for the device).
8. Repeat steps 4-7 for other cylinders. The pressure should not be lower than 1.0 MPa and should not differ in different cylinders by more than 0.1 MPa. Low compression in individual cylinders can result from loose valves in the seats, damaged cylinder head gaskets, broken or burnt piston rings. Low compression in all cylinders indicates worn piston rings.
9. To find out the causes of insufficient compression, pour about 20 cm3 of clean engine oil into the cylinder with reduced compression and measure the compression again. If the readings of the compression gauge have increased, the most likely malfunction of the piston rings. If the compression value remains unchanged, it means that the valve plates do not fit snugly against the seats or the cylinder head gasket is damaged.
Note. The cause of insufficient compression can also be found out by supplying compressed air to the cylinder in which the piston is previously set to TDC of the compression stroke. To do this, remove the tip from the compression gauge and attach the compressor hose to it. Insert the tip into the spark plug hole and inject air into the cylinder at a pressure of 0.2–0.3 MPa. To prevent the engine crankshaft from turning, engage a higher gear and brake the vehicle with the parking brake. Exit (a leak) air 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 expansion tank neck in the form of bubbles or into an adjacent cylinder, which is detected by a characteristic hissing sound.