Compression is checked with a special device - a compression gauge.
There are options for compression meters similar to those shown in the photo..
...and also those 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.
WARNING: Important conditions for the correctness of the readings during the compression test 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. Remove the decorative engine cover (see Removal and installation of a decorative casing of the engine).
3. Reduce the pressure in the engine power system (see Reducing the pressure in the engine supply system). Do not install the fuel pump circuit fuse in the central mounting block.
4. On 2.0L and 2.3L Duratec-HE engines, disconnect the wiring harness connectors from the ignition coils.
5. Turn out bolts of fastening of coils of ignition …
6.... and remove the ignition coils from the spark plugs.
7. Unscrew and remove the spark plugs.
8. On a 1.6L Duratec Ti-VCT engine, remove the wire ends from the spark plugs, remove all spark plugs (see Spark Plug Replacement and Maintenance) …
9.... and disconnect the low-voltage wire harness connector from the ignition coil, thereby disabling the ignition system.
WARNING: Turning the engine crankshaft with a starter with the tips of the high-voltage wires disconnected and the ignition coil not disconnected can lead to breakdown of its high-voltage circuit.
10. Screw the compression gauge into the spark plug hole of the cylinder to be checked.
11. Press the accelerator pedal to the stop so that when the crankshaft is turned by the starter, the throttle valve is fully opened in the engine cylinder purge mode.
12. 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: The crankshaft must rotate at 180-200 min-1 or higher, but no more than 350 min-1 to obtain a correct compression gauge reading.
13. Write down the readings of the compression gauge..
14….and set its pointer to zero by pressing the air release valve.
NOTE: Other designs of compression gauges may reset the readings in other ways (in accordance with the instructions for the device).
15. Repeat steps 10-14 for the rest of the 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.
16. To find out the causes of insufficient compression, pour about 20 ml 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 remains unchanged, it means that the valve plates do not fit snugly against their seats or the cylinder head gasket is damaged.
GOOD ADVICE: The cause of insufficient compression can also be found by supplying compressed air to a cylinder in which the piston is pre-set at TDC on 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.
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