Compression (pressure at the end of the compression stroke) in the cylinders is the most important indicator for diagnosing the condition of the engine without disassembling it.
By taking its average value and by taking the difference in values in individual cylinders, one can fairly accurately determine the degree of overall wear of the engine's connecting rod and piston components and identify faults in this group and valve train components.
Compression is checked with a special device - a compression gauge, which is now readily available at large auto parts stores.
This is what it looks like Compression gauge. There are compression gauges that have a rubber tip instead of a spark plug, instead of a threaded nipple.
When checking compression, these gauges simply press firmly. To ensure accurate readings when checking the spark plug hole, the starter and its electrical circuits are in good working order, as well as a fully charged battery.
You will need a 16 mm socket wrench to remove the spark plugs.
Start the engine and warm it up to operating temperature.
Stop the engine and disconnect the low-voltage wiring harness connector from the ignition module.
Cranking the engine with the starter while the high-voltage wires are disconnected and the ignition module is not disconnected may cause a breakdown in its high-voltage circuit.
Remove all spark plugs.
Disconnect the fuel pump by removing its relay in the mounting block installed in the engine compartment.
Screw the compression tester into the spark plug hole of the cylinder being checked.
Press the accelerator pedal Fully open the throttle valve.
Activate the starter and crank the engine until the cylinder pressure stops increasing. This corresponds to approximately four compression strokes.
To obtain accurate compression readings, the crankshaft should rotate at 180-200 rpm or higher, but no more than 350 rpm.
Record the compression readings from the compression gauge. Set its needle to zero by pressing the air bleed valve.
For compression gauges of other designs, readings may be reset in other ways (in accordance with the device's instructions).
Repeat steps 5-8 for the remaining cylinders.
The pressure should be at least 1.0 MPa and should not vary by more than 0.1 MPa between cylinders.
Low compression in individual cylinders may result from loose valve seats, damaged head gaskets, or broken or burned piston rings.
Low compression in all cylinders indicates worn piston rings.
To determine the cause of low compression, add approximately 20 cm³ of clean engine oil to the cylinder with low compression and measure the compression again.
If If the compression gauge readings have increased, piston ring failure is most likely.
If the compression reading remains unchanged, this indicates a loose valve seat or a damaged cylinder head gasket.
The cause is not clear enough Compression pressure can also be determined by injecting compressed air into the cylinder with the piston preset at TDC on the compression stroke.
To do this, remove the compression tester tip 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 high gear and apply the parking brake.
Air leaking through the throttle body indicates an intake valve leak, while air leaking through the muffler indicates an exhaust valve leak.
If the cylinder head gasket is damaged, air will escape through the expansion tank filler neck as bubbles or into the adjacent cylinder, which is detected by a characteristic hissing sound.