Compression (pressure at the end of the compression stroke) in the cylinders is the most important indicator of the condition of the engine without disassembling it

By the average value and by the difference in values in individual cylinders, it is possible to accurately determine the degree of wear of the parts of the cylinder piston group of the engine and identify its malfunctions and parts of the valve mechanism

Compression is checked by a special device - a compressor

Checking the compression in the engine cylinders of the Mazda 6

This is how the compressor looks like

There are compressor meters that have a rubber tip instead of a threaded fitting for screwing in instead of a spark plug

For the correctness of the readings during the test, it is necessary that the starter and battery are in good condition

Start the engine and warm it up to operating temperature

Reducing the pressure in the engine power system, for this:

Disconnect the negative battery terminal

Checking compression in the engine cylinders of a Mazda 6Checking compression in Mazda 6 engine cylinders

Turn off the fuel pump by removing the fuse or the fuel pump relay in the mounting block located in the engine compartment

On the back cover of the fuse and relay block there is a diagram of the location of the fuses and relays

Connect the negative terminal of the battery

Start the engine and let it run until the engine stops completely

Turn off the ignition. Now there is no fuel pressure in the fuel ramp.

Do not install the fuse of the fuel pump circuit in the central mounting block!

Removing the decorative engine cover

Checking compression in the engine cylinders of a Mazda 6Checking compression in Mazda 6 engine cylinders

Disconnect the pads of the wiring harnesses from the ignition coils

Unscrew the bolts securing the ignition coils

Checking compression in the engine cylinders of a Mazda 6Checking compression in Mazda 6 engine cylinders

Removing the ignition coils from the spark plugs

Unscrewing all the candles

Checking compression in the engine cylinders of a Mazda 6

Screw in or insert a compressor meter into the candle hole of the cylinder being tested

The assistant presses the gas pedal all the way so that when the crankshaft is cranked by the starter, the throttle valve fully opens in the engine cylinder purge mode

Turn on the starter, and turn the crankshaft of the engine until the pressure in the cylinder stops increasing. This corresponds to about four compression cycles

To obtain the correct readings of the compressor, the crankshaft must rotate at a speed of 180-200 min-1, or higher, but not more than 350 min-1.

Checking compression in the engine cylinders of a Mazda 6Checking compression in Mazda 6 engine cylinders

We record the readings of the compressor

After that, set the arrow to zero by pressing the air release valve

Repeat the operations for the remaining cylinders

The pressure must be at least 1.36 MPa (13.6 kgf/cm2) and must not differ in different cylinders by more than 0.1 MPa

Reduced compression in individual cylinders may occur:

- as a result of loose fitting of valves in the seats;

- damage to the cylinder head gasket;

- breakage or burnout of piston rings.

Reduced compression in all cylinders indicates wear of piston rings

To find out the cause of insufficient compression, you need to pour 20 ml of pure engine oil into a cylinder with reduced compression and measure the compression again

Checking compression in the engine cylinders of a Mazda 6

If the compressor readings have increased, the piston rings are likely to malfunction

If the compression value remains unchanged, it means that the valve plates do not fit tightly to their seats or the cylinder head gasket is damaged

The reason for insufficient compression can also be found out by supplying compressed air to the cylinder in which the piston is pre-installed in the TDC of the compression stroke.

To do this, remove the tips from the compressor meter and attach the compressor hose to it.

Insert the tip into the candle hole and supply air to the cylinder at a pressure of 0.2-0.3 MPa.

To prevent the engine crankshaft from turning, turn on the upper gear and brake the car with the parking brake.

The air outlet (leakage) through the throttle assembly indicates the leakiness of the intake valve, and through the muffler - the leakiness of 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