Case IH 3906 – ENGINE: Injector: Number of injections is limited by quantity balance of high pressure pump

This code indicates that the high-pressure fuel pump is failing to supply enough fuel volume or pressure to maintain the requested common rail pressure. To prevent the engine from stalling or the rail pressure from dropping below critical levels, the Engine Control Unit (ECU) actively intervenes by limiting the number of injection events per cycle, resulting in an engine derate.

• skid steer
• Steiger
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• Maxxum
• Farmall
• Axial-Flow 7120
• Axial-Flow 8120
• Axial-Flow 9120
• SR220
• SV250
• TR320
• TV380

1. 1

   Check the low-pressure fuel supply to the inlet of the high-pressure pump to rule out a clogged fuel filter or failing electric lift pump.

2. 2

   Test the Fuel Metering Unit (FCA / M-Prop) by unplugging it to see if the pump defaults to maximum rail pressure, indicating a sticking actuator.

3. 3

   Inspect the Pressure Relief Valve (PRV) on the common rail for premature fuel leakage back to the return line.

4. 4

   Perform a return flow test on the fuel injectors to check for excessive leak-off from worn injectors.

1. 1

   Read and Verify Codes

   Connect the EST (Electronic Service Tool) to verify code 3906. Look for accompanying codes like 3910 (Fuel metering unit loose contact) or 3915/3916 (Pressure Relief Valve issues) which can pinpoint the exact failure point.

2. 2

   Service the Fuel Filters

   Replace the primary and secondary fuel filters and bleed the system to ensure no air or debris is restricting the low-pressure circuit. Test run the machine.

3. 3

   Replace the Fuel Metering Unit

   If unplugging the FCA allows the pump to build pressure, or if the valve is found to be sticking/slow to respond, remove the FCA from the high-pressure pump body and install a new one.

4. 4

   Replace the High-Pressure Fuel Pump

   If the low-pressure supply is adequate, the FCA is functioning, and there are no leaks at the PRV or injectors, the high-pressure pump has failed internally and must be replaced. Remove the pump (driven by the engine timing gear/belt) and install the replacement unit.

5. 5

   Clear Codes and Test

   Use the EST service tool to clear the fault codes and test the machine under load to ensure rail pressure meets the ECU's requested parameters.