Fix BMW engine stall after starting

The car owner told us his BMW X3 sometimes engine stall after starting, the car is year 2017 with N20 engine F25 chassis.

Repair process:

1. To confirm the fault symptom, the test drive was started normally several times, and diagnose scanner got no error code. The vehicle is stopped for about 3 minutes, and the engine is automatically shut off after starting. After that, tried many times the car didn’t start, but the starter worked.

2. Possible cause: fuel system failure (include High-pressure fuel system and low-pressure fuel system) and other failure. High-pressure fuel system can caused by injector and high pressure pump, low-pressure system fuel system can caused by low-pressure fuel pump does not pump fuel, system jam, empty fuel tank, fuel pump module failure.

3. Start from inspect low-pressure system, install fuel pressure gauge, read the low pressure 1.4bar, it’s abnormal.


4. Read low pressure fuel pump data: the voltage 9.4V and the current is 8A, less than the normal value of 10A, indicating that the fuel pump load is low.

5. Check the function of the drainage pump by disassembling the fuel pump, the inspections are normal, but the oil pressure is low. Is there any leak in the pipeline?

6. We put the fuel pump in the fuel tank and started the vehicle to observe the fuel pump and pipeline. It was found that the pressure relief valve kept leaking oil at startup.


7. Continue to check the pressure relief valve and find that the internal O-ring is broken. After the O-ring is replaced, the test is normal, the low-pressure oil pressure is 6bar, and the low-pressure current is 10A.

8. Refer to fuel supply system diagram. The fuel direction is: fuel tank → low-pressure fuel pump → fuel filter → high-pressure fuel rail (excess fuel → pressure relief valve). Pressure relief valve is responsible for preventing the fuel system oil pressure from becoming too high. Generally, start to release at 5bar. The pressure relief valve is often open, resulting in oil pressure in the low-pressure system cannot be established. Especially after a few minutes of parking, the system oil pressure has been drained and it cannot be started.


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