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 the average compression value and by the difference in values ​​in individual cylinders, it is possible to determine with a sufficient degree of accuracy the degree of general wear of the parts of the connecting rod and piston group of the engine, to identify malfunctions of this group and parts of the valve mechanism.

Checking compression in engine cylinders

Compression is checked with a special device, a compression gauge, which is now freely available at major auto parts stores.

This is what the compression gauge looks like. There are variants of compression gauges that have a rubber tip instead of a threaded fitting for screwing in instead of a spark plug.

Such compression meters, when checking compression, are simply strongly pressed against the spark plug hole.

The important conditions for the correctness of the readings when checking the compression are the serviceability of the starter and its electrical circuits, as well as the full charge of the battery.

You will need a 21" socket wrench to remove the spark plugs.

Start the engine and warm it up to operating temperature.

Checking compression in engine cylinders

Stop the engine, disconnect the low voltage wire harness connector from the ignition module.

Warning. Cranking the engine with the starter while the high-voltage wires are disconnected and the ignition module is not disconnected can lead to breakdown of its high-voltage circuit.

Checking compression in engine cylinders

On the ZMZ-40524 engine, remove the ignition coils from the spark plugs, having previously disconnected the wiring harness pads from them.

Checking compression in engine cylinders

Remove all candles

Checking compression in engine cylinders

Disable the fuel pump by removing the wiring harness from the fuel pump relay terminals

Screw the compression tester into the spark plug hole of the cylinder to be checked.

Depress the accelerator pedal fully to fully open the throttle.

Engage the starter and crank the engine with it until the pressure in the cylinder stops increasing. This corresponds to approximately four compression cycles.

Checking compression in engine cylinders

After recording the compression gauge readings, set its arrow to zero by pressing the air release valve.

For compression meters of a different design, readings can be reset in other ways (in accordance with the instructions for the device).

Repeat these steps for the rest of the cylinders.

The pressure must be at least 660 kPa (6.65 kgf/cm2) and must not differ in different cylinders by more than 98 kPa (1 kgf/cm2).

Lower compression in individual cylinders can result from loose valve seats, damaged cylinder head gaskets, broken or burnt piston rings.

Lower compression in all cylinders indicates worn piston rings.

To determine the cause of insufficient compression, fill the decompressed cylinder with about 20 cm3 of clean engine oil and measure the compression again.

If the compression gauge goes up, the piston rings are most likely to be bad.

If the compression remains unchanged, then this indicates a loose fit of the valve plates to their seats or damage to the cylinder head gasket.

Helpful advice. The cause of insufficient compression can 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 tester and attach the compressor hose to it.

Insert the tip into the spark plug hole and blow air into the cylinder at a pressure of 200-300 kPa.

In order to prevent the engine crankshaft from turning, engage a higher gear and brake the car with the parking brake.

The exit (leakage) of air through the throttle assembly indicates a leak in the intake valve, and through the muffler - a leak in the exhaust go.

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.

As the compression rings are cut, air will escape through the dipstick.