101902 Performing Compression Pressure Check on DFI Engines
WM 101902 Performing compression pressure check on DFI engines
Preliminary work
1. Open the bonnet.
2. Open lid of central electrics box. To do this, loosen the four fastening screws. -arrows-
Central electrics box
WARNING
Risk of electric shocks from high-voltage ignition system and danger of damage to ignition rod modules!
- Connected ignition rod modules can be damaged.
-> Always remove the ignition rod module fuse.
-> Always disconnect ignition rod module electrical connections.
CAUTION
Risk of damage to components that affect the exhaust system!
- Unburnt injected fuel enters the exhaust system.
-> Prevent the fuel from injecting by removing the fuse.
3. Pull out the fuses for the ignition rod modules and the injection system. For latest assignment, see wiring diagram.
4. Remove spark plugs. -> 287020 Removing and installing spark plugs on DFI engines - section on "Removing" Service and Repair
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 of V8 DFI engines is much higher than that of 4.5-litre intake manifold injection engines. The old compression tester Moto-Meter Compression Recorder 1065 is not able to display the measuring ranges and will be irreparably damaged.
- 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.
Test evaluation and test prerequisites
Information
The 4.8-litre V8 naturally aspirated engine has a compression of 12.1 : 1. The 4.8-litre V8 turbo engine has a compression of 10.1 : 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.
- The throttle valve must remain open during the test.
- The engine should have a crank drive that has been run in. Mileage at least 2000 km.
- The vehicle voltage must remain more or less constant during the test (connect a charger).
- The vehicle voltage must not drop below 12 V.
- 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.
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 pressure indication of absolute values as this 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).
Testing engine compression
Information
- This process should not be interrupted.
- The vehicle voltage must not drop below 12 V during the test.
- The throttle valve must always remain in the same position throughout the test. A mechanical pressing-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 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. Connect pressure tester to the corresponding cylinder via the extensions or adapters.
2. Get a second person to actuate the starter. The test for this cylinder is complete once the test equipment's pointer stops climbing. Keep the accelerator pedal fully depressed.
3. Write down or save the measured value reached. Deflate the pressure on the test equipment. Follow the operating instructions for the equipment.
4. Move the scale on to another cylinder if necessary.
5. Then check the next cylinder according to number sequence (not according to firing order).
Subsequent work
1. Fit spark plugs. -> 287020 Removing and installing spark plugs on DFI engines - section on "Installing" Service and Repair
2. Fit the fuses for the ignition rod modules and the injection system.
3. Fit lid of central electrics box. -arrows-
Central electrics box
4. Read out the fault memory and erase it if necessary.