Compression (compression stroke end pressure) in cylinders - the most important indicator for diagnosing the condition of the engine 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 parts of the connecting rod and piston group of the engine and to identify faults in this group and parts of the valve mechanism.
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 spark plug instead of a threaded fitting for screwing in. When checking compression, such compression testers are simply pressed hard 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 the full charge of the battery.
You will need...
...a 16 mm wrench for removing spark plugs.
Note. The procedure for checking the compression in the cylinders is shown using the K7J and K7M engines 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 (see "Checking, removing and installing ignition coils of the K4M engine").
1. Start the engine and warm it up to operating temperature.
[Text published on the website: renaultbook]
2. Stop the engine, disconnect the low voltage wire harness connector from the ignition module.
Warning: Cranking the engine with the starter motor with the high-voltage wires disconnected and the ignition module not disconnected may cause a breakdown in its high-voltage circuit.
3. Remove all spark plugs (see "Replacement and maintenance of 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 the pressure in the cylinder stops increasing. This corresponds to approximately four compression strokes.
Note: To obtain correct compression tester readings, the crankshaft must rotate at 180-200 min⁻¹ or higher, but not more than 350 min⁻¹.
8. After recording the compression gauge readings, set its arrow to zero by pressing the air release valve.
Note: Compression gauges of other designs may reset in other ways (in accordance with the instructions for the device).
9. Repeat steps 5-8 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.
10. 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 value remains unchanged, this indicates that the valve plates do not fit tightly to their seats or that the cylinder head gasket is damaged.
Helpful advice. 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.