BMW E39 M57 rail pressure fluctuates
E39 M57: why does the rail pressure fluctuate and the idle jerk?
The cause is often a faulty rail pressure sensor, a poor sensor connector or the pressure regulator. Injector leak-by and the pressure regulator seals are also worth checking. If consumption and smoke increase during brisk driving, the fault is most likely in the injectors.
What does a fault in the rail pressure sensor or its connector cause?
A faulty sensor or poor contacts in the connector can cut off injection completely or partly. The car then bucks and stutters or the idle hunts. First check that the pins of the sensor connector are firmly seated before replacing parts.
Does replacing the pressure regulator help with fluctuating rail pressure?
In some cars the fluctuation and uneven idle stopped after replacing the pressure regulator. A new quantity control valve has also fixed a hunting idle on a warm engine. A blocked or slow regulator makes the pressure correct slowly and overshoot the target.