2.2.56. SPN 157, FMI 1, Rail pressure monitoring: Positive pressure deviation, not possible to reduce rail pressure.ΒΆ

Description

The EEM4 monitoring system has detected a positive deviation in the rail pressure. The measured rail pressure is not decreasing according to the setpoint value to the MPROP. Typically it indicates a clogged diesel fuel return line (to the tank), a leakage in the MPROP, or an MPROP stick with an open condition.

Possible causes are:

  • The fuel return line back to the tank is clogged.
  • The high pressure pump is not working correctly.
  • The rail pressure sensor wiring is defective.

Reaction in EEM

The FC is stored and the warning lamp is activated. Engine power is heavily reduced (Degradation Level 3). The CAN message indicates an active fault. The engine functions dependent on the rail pressure are not active.

Solution

First save the error log and then do the procedure that follows:

  • A temporary pressure peak (breakdown) during engine running. For example an abnormal load drop/fuel quantity drop during running. If the fault does not activate within the next 10 operating hours, the cause was probably a too sensitive diagnostic function.

Turn the ignition key off and back on. Start the engine; if the fault code is still active when the engine is running, check the following paths:

  • Examine the fuel return line. During engine running, the constant diesel fuel flow should return to the fuel tank (minimum 3 L/h).

To detect a possible return line problem:

  • Return the diesel fuel temporarily to a canister from the engine return line connector. Check the correct engine return line location from the engine/vehicle service manual. If the fault does not activate anymore, the cause was probably a clogged return line.
  • Examine the high pressure pump: Look for possible high pressure pump faults and inspect them accordingly (e.g. faults with the wiring and the connectors).
  • Test the high pressure pump operation with the Service Tool test function, or have the pump inspected by an authorized Bosch service dealer.
  • Examine the tightness of the injector side feed pipes. The correct torque tightening and operating instructions can be found in the engine repair manual.
  • Examine the rail pressure sensor wiring, the connectors and the contact surfaces of the connector pins (possible oxidation).
  • Make sure that the 5 V power supply to the rail pressure sensor is correct.
  • Try the operation with another rail pressure sensor to see if the sensor is defective.
  • In case of a damaged high pressure pump, rail pressure sensor or a wiring harness, replace the damaged parts according to the instructions in the service manual.

See Rail pressure sensor for reference.