3.2.278. SPN 520207, FMI 31, Rail Pressure control: Pressure BELOW normal in limp home modeΒΆ

Description

The measured rail pressure is below the normal operating range in the limp home mode. The warning limit depends on the engine speed.

Possible causes are:

  • No fuel in the tank (a defective instrument panel).
  • The filter is choked (impurities, freezing, etc.).
  • A leak / air in the system (low fuel pressure side).
  • The fuel lift pump is defective.
  • A fuel leakage from the high pressure side.
  • The injection nozzle is stuck in the open position.
  • The high pressure pump is not working correctly.
  • The high pressure pump is worn.
  • A leaking pressure control valve.
  • The rail pressure sensor wiring is defective.
  • The rail pressure sensor power supply is not correct.
  • The rail pressure sensor 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 active fault. The engine functions dependent on the rail pressure are not active.

Note: Due to the very high injection pressure, there is a serious risk for injury during engine running, if the high pressure side has a leakage.

Solution

First save the error log and then erase it. Restart the ECU after erasing the error log. If the fault occurs again during the next driving cycle, check the following paths:

  • Check the fuel level from the tank. (The gauge is possibly showing a wrong level.)
  • Check if the fuel filters are choked (impurities, freezing, etc.) and replace the filters (pre- and main filters).
  • Check the fuel low pressure side for a possible leak/air in the fuel system.
  • Check the high pressure pump control wiring, the connectors and the contact surfaces of the connector pins (possible oxidation).

This fault will typically activate, when there is a fuel supply problem on the fuel low pressure side. If the fault is continuously active, see solution 2.

Solution 2

  • Stop the engine. Check the engine high pressure pipes etc. visually for a possible leakage.
  • Check the high pressure pump: Look for possible high pressure pump faults and inspect them accordingly (e.g. faults with the wiring and the connectors).
  • Check the high pressure pump operation with the Service Tool test function, or have the pump inspected by an authorized Bosch service dealer.
  • Check the rail pressure sensor wiring, the connectors and the contact surfaces of the connector pins (possible oxidation).
  • Check the 5 V power supply to the rail pressure sensor.
  • Check the operation with another rail pressure sensor to see if the sensor is defective.

NOTE: The rail pressure sensor removal, inspection and replacement should be performed only by qualified injection system service personnel. Special instructions may apply.

See Rail pressure sensor for reference.