Diesel Fuel Injection
Fuel Injectors, Checking
There are 4 ways to check the injectors. Refer to one of the following:
• => [ Injector Correction Values, Check and Adjust ]
• => [ Injector Return Quantity, Checking with the Engine Running ]
• => [ Injector Return Quantity, Checking at Starter Speed ]
• => [ Open Injector, Checking ]
Injector Correction Values, Check and Adjust
Special tools, testers and auxiliary items required
• Vehicle Diagnostic Tester
• The "injector correction value" corrects the fuel injection quantity for each cylinder individually in the common rail system over the entire characteristic map.
• The 7 digit correction values - 1 - are stamped on each injector (example). The stamped values can be letters and/or numbers (ASCII Code).
- When replacing an injector, the correction value for the new injector must be programmed into the Engine Control Module (ECM).
The procedure for doing the adaptation is described in Guide Fault Finding.
Check all the other injectors as well regarding their "injector correction value" to make sure all comparison values have been entered correctly. If the correct comparison value have been stored in the ECM , then these comparison values must never be entered again.
Injector Return Quantity, Checking with the Engine Running
• Follow the guidelines for a clean working environment when working on the fuel system. Refer to => [ Clean Working Conditions and Fuel System Work Instructions ] Clean Working Conditions and Fuel System Work Instructions.
• Always follow these rules before and during work.
Special tools, testers and auxiliary items required
• Return Flow Meter (VAS 6684)
Each injector has a small fuel return quantity. If the return quantity is relatively large (in comparison to the quantities from the other injectors), then this is the injector that is faulty.
- Remove the engine cover. Refer to => [ Engine Cover ] Engine Cover.
- Clean all of the return line connections (for example, with a commercially available refrigerant cleaner) before removing.
- Dry all of the cleaned components.
- Remove the return line connections from the 6 fuel injectors on both of the cylinder banks. To do this, press the connection downward at the retaining straps and pull the center piece upward in the - direction of the arrow - to release it.
• The illustration shows cylinder bank 2.
• The coolant expansion tank must be removed for cylinder bank 2. The coolant hoses remain connected.
• Maintain clean working conditions. Dirt and contaminants must not enter the return lines and the connections on the fuel injectors.
- Connect the return flow meter (VAS 6684) as shown.
Connecting the Return Flow Meter
• All 6 injectors may be connected at the same time or each cylinder bank is measured separately.
- Start the engine and let it idle for a few minutes.
Do not use the accelerator pedal during the test. The engine always must be idling.
If the return lines are removed, the increased engine speed will lead to injector damage.
Return Quantity Values
• 1. Fuel injectors are OK. Return quantity of all of the fuel injectors is almost identical.
• 2. Fuel injector for cylinder 3 is not OK. The return quantity exceeds three times the lowest measured return quantity.
• There is a mechanical error in the fuel injector, if the return quantity is more than three times that of the lowest measured return quantity. This fuel injector needs to be replaced.
- If necessary, replace the fuel injector(s) that are diagnosed as defective, refer to => [ Fuel (Piezo) Injectors ] Service and Repair.
- Inspect the O-ring for the fuel return line connection at the injector for damage and deformation.
Is there identifiable O-ring damage or deformation:
- Replace the O-ring for the fuel return connection.
- Coat the O-ring for the fuel return connection and the fuel return connection with assembly oil or clean engine oil.
- Roll the new O-ring with extreme care onto the fuel return connection. Check that it is seated correctly.
- Carefully press the return line connection onto the injector. The connection must engage audibly. Then press the center piece down carefully.
- Reconnect the electrical connector for the fuel pressure regulator valve.
- Erase the fault stored in the Diagnostic Trouble Code (DTC) memory, refer to the vehicle diagnostic tester.
- Perform the fuel system leak test. Refer to => [ Fuel System, Checking for Leaks ] Fuel System, Checking for Leaks.
Injector Return Quantity, Checking at Starter Speed
If the engine will not start, the injector return quantity can also be checked at starter speed.
Special tools, testers and auxiliary items required
• Return Flow Meter (VAS 6684).
• Follow the guidelines for a clean working environment when working on the fuel system, refer to => [ Clean Working Conditions and Fuel System Work Instructions ] Clean Working Conditions and Fuel System Work Instructions.
• Always follow these rules before and during work.
Each injector has a small fuel return quantity. If the return quantity is relatively large (in comparison to the quantities from the other injectors), then this is the injector that is faulty.
- Remove the engine cover. Refer to => [ Engine Cover ] Engine Cover.
- Clean all of the return line connections (for example, with a commercially available refrigerant cleaner) before removing.
- Dry all of the cleaned components.
- Remove the return line connections from the 6 fuel injectors on both of the cylinder banks. To do this, press the connection downward at the retaining straps and pull the center piece upward in the - direction of the arrow - to release it.
• The illustration shows cylinder bank 2.
• The coolant expansion tank must be removed for cylinder bank 2. The coolant hoses remain connected.
• Maintain clean working conditions. Dirt and contaminants must not enter the return lines and the connections on the fuel injectors.
- Connect the return flow meter (VAS 6684) as shown.
Connect the Return Flow Meter
• All 6 injectors may be connected at the same time or each cylinder bank is measured separately.
- Disconnect the connector - 1 - from the fuel pressure regulator valve.
• This prevents fuel from being injected during the starting procedure.
- Crank the starter 3 times. Allow 20 second between each cranking attempt. Otherwise the starter could overheat.
Return Quantity Values
• 1. Fuel injectors are OK. Return quantity of all of the fuel injectors is almost identical. Specified value: approximately 0 ml
• 2. Fuel injector for cylinder 3 is not OK. The return quantity exceeds three times the lowest measured return quantity.
• There is a mechanical error in the fuel injector, if the return quantity is more than three times that of the lowest measured return quantity. This fuel injector needs to be replaced.
- If necessary, replace the fuel injectors that are diagnosed as defective, refer to => [ Fuel (Piezo) Injectors ] Service and Repair.
- Inspect the O-ring for the fuel return line connection at the injector for damage and deformation.
Is there is identifiable O-ring damage or deformation:
- Replace the O-ring for the fuel return connection.
- Coat the O-ring for the fuel return connection and the fuel return connection with assembly oil or clean engine oil.
- Roll the new O-ring with extreme care onto the fuel return connection. Check that it is seated correctly.
- Carefully press the return line connection onto the injector. The connection must engage audibly. Then press the center piece down carefully.
- Reconnect the electrical connector to the fuel pressure regulator valve.
- Erase the fault stored in the Diagnostic Trouble Code (DTC) memory, refer to the vehicle diagnostic tester.
- Perform the fuel system leak test. Refer to => [ Fuel System, Checking for Leaks ] Fuel System, Checking for Leaks.
Open Injector, Checking
Open injector means: the nozzle needle does not close all of the way and fuel leaks into the cylinder.
• Follow the guidelines for a clean working environment when working on the fuel system. Refer to => [ Clean Working Conditions and Fuel System Work Instructions ] Clean Working Conditions and Fuel System Work Instructions.
• Always follow these rules before and during work.
Special tools, testers and auxiliary items required
• Hand Vacuum Pump (VAS 6213)
• Create an Adapter Hose out of a Old Return Line.
- Erase the fault stored in the Diagnostic Trouble Code (DTC) memory using the vehicle diagnostic tester.
- Remove the engine cover. Refer to => [ Engine Cover ] Engine Cover.
- Clean all connections, for example with a commercially available detergent, before removing.
• Maintain clean working conditions. Contamination must not enter the fuel system.
• Check one cylinder after another.
- Dry all cleaned components.
Begin with cylinder 1.
- Remove the return line connections from the fuel injectors by pressing both retaining straps down while pulling up - arrow - on the center piece to release.
- Connect the clean adapter hose that has been cleaned out using compressed air to the return line connection on the injector that is being checked.
- A vacuum pressure of 500 mbar (7.26 psi) is applied using the hand vacuum pump (VAS 6213)
If the injectors are OK, then the pressure will remain constant for 30 seconds.
If the injectors fail, then the pressure will drop to 0 bar (0 psi) within 2 to 3 seconds.
Repeat, if necessary, and pay special attention to the loss in pressure on the hand vacuum pump (VAS 6213).
- Replace the defective injectors, refer to => [ Fuel (Piezo) Injectors ] Service and Repair.