Compression (compression stroke end pressure) in cylinders - the most important diagnostic indicator of the engine, carried out 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's piston group parts and identify faults in this group and valve mechanism parts.
To check, use a compression tester, which can be purchased at auto parts stores. There are compression gauges on sale, both threaded, which have a threaded nipple installed for screwing into place of the spark plug, and with a rubber tip, which are simply pressed firmly against the spark plug hole.
The conditions for correct readings when checking compression are the serviceability of the starter and a fully charged battery.
The procedure for checking the compression in the cylinders is shown using the K7M engine as an example. Check the compression of the K4M engine in a similar manner.
The difference is that the four ignition coils must first be removed from the K4M engine spark plugs.
1. Start the engine and warm it up to operating temperature.
2. Stop the engine, disconnect the low voltage wire harness connector from the ignition module.
Cranking the engine with the starter while the high-voltage wire ends are disconnected and the ignition module is not disconnected may cause a breakdown in its high-voltage circuit.
3. Remove all spark plugs.
4. Disconnect the fuel pump by removing its relay in the mounting block installed in the engine compartment of the vehicle.
5. Screw the compression gauge into the spark plug hole of the cylinder being tested.
6. Press the accelerator pedal all the way down to fully open the throttle valve.
7. Turn on the starter and turn the engine crankshaft until. until the pressure in the cylinder stops increasing. This corresponds to approximately four compression strokes.
To obtain correct compression tester readings, the crankshaft must rotate at a speed of 180-200 min ' or higher, but not more than 350 min '.
8. After recording the compression gauge readings, set its arrow to zero. (The readings can be reset in different ways depending on the design of the device).
9. Repeat steps 5-8 for the remaining cylinders. The pressure should not be lower than 1.0 MPa and should not differ in different cylinders by more than 0.1 MPa. Reduced compression in individual cylinders may occur as a result of loose valve seating, damage to the cylinder head gasket, or broken or burnt piston rings. Low compression in all cylinders indicates wear of the piston rings.
10. To determine the reasons for insufficient compression, pour about 20 ml of clean motor oil into the cylinder with low compression and measure the compression again. If the compression gauge readings have increased, the piston rings are most likely faulty. If the compression value remains unchanged, this indicates that the valve plates are not tightly seated against their seats or that the cylinder head gasket is damaged.
The cause of insufficient compression can also be determined by supplying compressed air to the cylinder, in which the piston is pre-set at 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 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. The output (leakage) of 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 the adjacent cylinder, which is detected by a characteristic hissing sound.