Compression check (BMW 5 Series E34)

The following measuring instrument is required:

• Compression measuring device.

Note: Diesel engine requires a device with a larger measuring range.

Compression should be checked every 80,000 km. On gasoline engines, the spark plugs should be removed. It is recommended to combine this work operation with changing the spark plugs. On diesel engines, the glow plugs are removed when checking compression.

The results of the compression test allow us to draw conclusions about the condition of the engine. Namely, the test allows us to draw a conclusion about the wear of the valves or pistons (piston rings), in addition, based on the results of the inspection, it is possible to determine whether the engine can be repaired by replacing individual parts or whether it requires a major overhaul.

The difference in compression of individual cylinders should not exceed 1.5 bar. Exceeding these values is a sign of defective valves, wear of piston rings or cylinder working surfaces. When the wear limit is reached, the engine is subject to major repairs or replacement.

The value of the measured compression is of secondary importance, since it also depends on the measuring device used. Compression on petrol engines should be at least 10 bar, and on diesel engines at least 20 bar.

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

Petrol engines

Turn off the ignition.

Remove the cover from the electronics unit in the engine compartment behind the right shock absorber by unscrewing four bolts.





Model 520i, 525i: Remove the main relay to prevent fuel injection during testing.

Models 530i, 535i: Remove relays 1 and 2.

Note: On vehicles with EML (engine power control system) remove only relay 1. Additionally, disconnect the wire from terminal 1 on the ignition coil.

Remove all spark plug tips. There are special pliers for this, for example, from HAZET 1849. However, these pliers can only be used to grasp the metal bushings of the tips.

Blow out the spark plug recesses in the cylinder head with compressed air and use a special spark plug wrench to remove all the spark plugs.

Turn the engine over a few times with the starter to remove carbon deposits.

Caution: The gearbox is in neutral position, the handbrake is applied.

Connect the compression tester to the spark plug hole in accordance with its operating instructions.



The assistant must press the gas pedal all the way down and keep his foot on the pedal in this position throughout the entire test.

Crank the engine approximately 8 revolutions until the reading on the gauge scale stops increasing.

check the compression in all cylinders in sequence and compare with the specified value.

Finally, tighten the spark plugs to a tightening torque of 25 Nm and connect the spark plug wires according to the ignition sequence 1-5-3-6-2-4.

Install the main relay.

Diesel engine

Test condition: Charged battery.

Unscrew the cover of the electronics unit on the right side of the engine compartment.

Remove the main glow relay and connector from the glow timing control unit.

Disconnect the electrical wires from the glow plugs and unscrew the spark plugs with the appropriate wrench head.

Instead of glow plugs, screw in a compression measuring device.

Turn the engine with the starter until the pressure stops increasing. Record the result. Repeat the test on all cylinders.

After checking, screw in and tighten the glow plugs to 25 Nm. Pre-coat the spark plug threads with copper paste "CRC".

Connect the wires to the glow plugs with a tightening torque of 5 Nm.

Insert the main relay, connect the connector to the glow plug control unit.

Close the electronics unit.