To check the compression of a diesel engine, a special compression tester is required, which is screwed into the threaded hole of the glow plugs. Checking the compression of a diesel engine is not described here.
The difference between the compression pressure values of individual cylinders should not exceed 1.0 bar for a petrol engine and 3.0 bar for a diesel engine.
If one or more cylinders have a higher pressure than the others, this indicates damaged valves, worn piston rings or damaged cylinder head gaskets. In this case, the engine must be repaired or replaced.
The pressure value is of secondary importance, as it depends, among other things, on the measuring device used. For petrol engines, it should not be less than 10 bar.
Examination
1. The condition for testing is a charged battery.
2. Turn off the ignition.
3. Disconnect the main relay of the electronic engine management system to prevent fuel injection during the test. The location of the control system relay is shown in the Specifications Chapter On-board electrical equipment.
If the engine control system relay does not disconnect, the output stage of the main control system relay may be damaged.
4. Remove all spark plugs, refer to Section Checking fluid levels.
5. Models 316i, 318i: Disconnect the multi-pin connector of the ignition coils by turning the knob "1" to the left.
6. Set the manual transmission to neutral, the automatic transmission to position "P". Apply the parking brake.
Do not lean over the engine if it is turning over with the spark plugs removed. There is a risk of injury due to the soot particles ejected!
7. Insert or screw the compression tester into the spark plug hole according to the operating instructions.
8. Ask the assistant to fully depress the gas pedal and hold it in this position for the entire duration of the test.
9. Turn the engine over approximately 8 revolutions until the pressure on the compression gauge stops increasing.
10. Thus, check all cylinders in sequence.
11. If the compression in one or more cylinders is below the wear limit, pour some engine oil through the spark plug hole and measure the pressure again with a compression gauge. If the pressure increases after this, this indicates wear of the piston rings and/or the cylinder running surface. If the pressure does not increase, then, as a rule, the valve or cylinder head gasket is damaged
12. Install the spark plugs, refer to Section Checking fluid levels.
13. Models 316i, 318i: Connect the multi-pin connector of the ignition coils while turning the handle to the right.
14. Install the engine management system main relay, refer to Chapter Specifications On-board electrical equipment.
Information taken from this resource BMWman