Compression Check: Testing and Inspection

WM 101902 Checking compression of cylinders

Tools and materials

Tools

Information

- A commercially available compression tester with a maximum pressure of at least 25 bar is required for the compression test on V8 DFI engines.

- The compression tester Moto-Meter Compression Recorder 1065 cannot display the measuring ranges and will be damaged irreparably.

- The compression tester used should be equipped with a removable adapter for the thread matching the spark plug recess (M14 x 1.25). A rubber cone that is pushed in is not permitted as the cone will be lifted by the high compression and this will falsify the measurement result.

- On compression testers with an analogue pointer, the measured value of the corresponding cylinder must be written down after each measurement.

- Always follow the operating instructions provided by the equipment manufacturer.

Checking compression of cylinders

- This process should not be interrupted.
- The vehicle voltage must not drop below 12 V during the test.
- The throttle must always remain in the same position throughout the test. A mechanical press-on tool is not permitted.
- The accelerator pedal must be fully depressed during the test.
- The throttle valve must remain open during the test.
- The vehicle voltage must remain more or less constant during the test (connect a charger).
- The fuses for the injection system and the ignition system must be removed during the test.
- The engine must be at operating temperature.
- Electrical loads switched off.
- Air conditioning switched off.

Information

The 4.8-litre V8 naturally aspirated engine has a compression ratio of 12.5 : 1. The 4.8-litre V8 turbo engine has a compression ratio of 10.5 : 1.

The 4.8-litre V8 naturally aspirated engine has higher compression values than the 4.8-litre V8 turbo engine. They are not comparable.

The measurement of two comparable V8 naturally aspirated engines can produce different pressure ranges. This is not a sign of wear or a defect.

Information

The compression test can only issue information regarding whether the cylinder being tested is OK or not OK. Further localisation of the cause of loss is achieved by a pressure loss test or a compression chamber endoscopy.

1. Open the bonnet.
2. Pull out the fuses for the ignition rod modules and the fuel injectors. For latest assignment, see wiring diagram.
3. Remove spark plugs. -> 287020 Removing and fitting spark plugs Service and Repair
4. Support start procedures. -> 2706IN External power connection, jump-lead starting Service and Repair

Test values =>
1) The compression values are heavily dependent on: Amount of deposits in combustion chamber, state of the combustion chamber seal (valves, piston rings, etc.), starter speed, engine temperature, engine mileage etc.
- No absolute pressure values displayed as these would not be meaningful.
- expected pressure ranges of 4.8-litre V8 naturally aspirated engine = approx. 18 to 25 bar (no wear limits). It is not a sign of wear or defects if the pressure exceeds or drops below these values.
- expected pressure ranges of 4.8-litre V8 turbo engine = approx. 15 to 18 bar (no wear limits). It is not a sign of wear or defects if the pressure exceeds or drops below these values.
- maximum difference between the cylinders in bar overpressure (within one measurement) = 3.0 (single permissible criterion for suspected wear). Further tests, such as pressure loss test or endoscopy, should follow. For example: a V8 naturally aspirated engine with 20.6 bar at cylinder 2 and 16.9 bar at cylinder 8 (wear suspected).