Compression check (BMW 5 Series E39)

Compression testing allows you to draw conclusions about the condition of the engine. Namely, it allows you to determine whether the valves or pistons are in good condition (piston rings), or they are worn out. In addition, the check shows whether the engine should be replaced or completely overhauled.

To check, you need a compression tester, which is available in specialized stores at a very low cost for gasoline engines.

Note: Diesel engines require a compression tester with an extended measurement range.

The most information is given by the difference in pressure in individual cylinders. It should not exceed 1.5 bar. If one or more cylinders have a pressure that differs from the pressure in other cylinders by more than 1.5 bar, this indicates a defect in the valves, wear of the piston rings or working surfaces of the cylinders. If the wear has reached the limit, the engine must be repaired or replaced.

For petrol engines, it is necessary to remove the spark plugs; it is recommended to combine this work operation with the scheduled replacement of spark plugs. In a diesel engine, the glow plugs are removed to check the compression.

The value of the displayed compression pressure is indirect in nature, since it depends, among other things, on the device used. This value for gasoline engines should be at least 10 bar, and for diesel engines - at least 20 bar.

A mandatory condition for checking is a charged battery.

To check compression, a gasoline engine must be warmed up to operating temperature, and a diesel engine must be cold.

Gasoline engine

Warm up the engine while driving the car until the coolant temperature gauge shows the normal operating temperature. Turn off the engine, turn off the ignition.

Remove the air microfilter housing located on the right side of the engine compartment to gain access to the electronics unit.



Unscrew the "arrow" fastening and remove the cover of the electronics unit.



To exclude fuel injection during testing, remove the main relay "1" of the DME system (electronic digital engine management system), "2" DME system fuses.

Remove all spark plugs, see Maintenance section.

Note: The gearbox must be in neutral and the handbrake must be applied. Turn the engine crankshaft a couple of times with the starter to throw out carbon and soot from the cylinder.

Press or screw the compression gauge into the spark plug hole in accordance with the operating instructions.

Ask an assistant to press the accelerator pedal all the way down and hold it with his foot throughout the entire test.

Turn the engine shaft approximately 8 revolutions until the arrow on the compression gauge scale stops showing an increase in pressure.

Check all cylinders in sequence and compare the obtained pressures with the prescribed values.

After this, install the spark plugs, see the section "Maintenance work".

Put the main relay back in place and close the electronics unit with the cover.

Install the microfilter housing to clean the air entering the passenger compartment.

Diesel engine

Remove the air microfilter housing located on the right side of the engine compartment to gain access to the electronics unit.

Remove the electronics unit cover.



520d, 530d: To prevent the engine from starting, disconnect the plug connector "1" of the camshaft pulse sensor.

525tde: Remove the preheating system relay. For location of parts, see page 60.

Unscrew the electrical wire from the glow plugs and remove the glow plugs using a suitable socket wrench.



Screw in a compression gauge instead of the glow plug.

Turn the engine shaft with the starter until the arrow on the compression gauge scale stops showing an increase in pressure. Record the measured pressure value. Check all cylinders sequentially.

After checking, screw in the glow plugs and tighten them to a torque of 25 Nm. Pre-treat the glow plug threads with CRC high-temperature copper grease, available at BMW service stations.

Tighten the electrical wire fastening to the glow plugs to a torque of 5 Nm.

Insert the pre-heating system relay.

Close the cover of the electronics unit. Install the microfilter housing to clean the air entering the cabin.