The compression is checked with a special device – a compression meter.
There are variants of compression gauges similar to the one shown in the photo…
...and also those that have a rubber tip installed instead of a spark plug instead of a threaded fitting for screwing in. Such compression gauges are simply pressed hard against the spark plug hole when checking compression.
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. Remove the decorative engine cover (see Removing and installing the decorative engine cover).
3. Reduce the pressure in the engine fuel system (see Reducing pressure in the engine fuel 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. Remove the ignition coil mounting bolts…
6….and remove the ignition coils from the spark plugs.
7. Unscrew and remove the spark plugs.
8. On the 1.6L Duratec Ti-VCT engine, remove the spark plug wire ends, unscrew all the spark plugs (see Replacing and servicing spark plugs)…
9….and 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.
10. Screw the compression gauge into the spark plug hole of the cylinder being tested.
11. 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.
12. 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.
13. Record the compression gauge readings…
14….and 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).
15. Repeat steps 10–14 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.
16. 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 supplying compressed air to 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 supply air to 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.