Manual - Chapter 5 - Fuel & Oil

11: Fuel pressure: testing

Note: Before starting work on any part of the fuel system, read the notes given in Section 2 of this Chapter concerning safety precautions.

  1. The only test of the fuel pump's and pressure regulator's performance is to measure the delivery pressure, as described below. Note however that if the fault occurs under a particular set of circumstances, for example only in very hot weather, these circumstances should be duplicated as closely as is reasonably possible when making the test. First relieve any residual fuel system pressure as described in Section 3.
  2. Referring to Section 5 of this Chapter disconnect the fuel feed hose at the fuel rail and connect a suitable pressure gauge between the hose and the rail; BMW's own gauge is available under part number 16 1 500
  3. Start the engine and allow it to idle while taking pressure readings. Since fuel pressure is not related to engine speed there is no point in running the engine any faster unless the fault occurs only at higher speeds or the pressure regulator performance is to be checked. In the latter case open and close the throttle sharply once or twice to note the effect of the differing intake vacuum pressures on the fuel pressure. If the pressure regulator is faulty, it must be renewed.
  4. The pressure reading obtained should be steady at approximately 2.5 bar (36 psi) although some BMW service literature does mention a figure of 2.2 bar (32 psi) at idle speed. If the reading obtained is significantly lower than that specified, check the tank hoses and pipes on the pump delivery side (Section 6), and the fuel filter (Section 8) before renewing the fuel pump as described in Section 9.
  5. If the pressure recorded is significantly higher than that specified, the pressure regulator and its vacuum hose should be checked (Section 12). If the pressure exceeds 4.7 bar (68 psi) indicating that the regulator safety valve has stuck, the pressure regulator should be renewed immediately.