BMW E34 fuel pump, relay and fuel pressure
E34 fuel pump does not run - where is the fault usually found?
Check the pump relay first, then the power supply and ground, and finally the pump and fuel filter. The relay is a common cause: when cold, the relay may fail to pull in. The pump on the Motronic-controlled M20 usually does not buzz when you switch the ignition on, but runs only when you try to start.
How do I test the E34 fuel pump relay and pump?
1. Switch the ignition on: relay terminals 30 and 86 must get +12 V and the relay must click. Pin 85 is the control unit ground. 2. Feed power to the pump directly from the battery. If the pump runs, the fault is in the relay or the wiring. 3. Route the pump's pressure hose into an empty bottle and switch the ignition on: the pump should pump fuel within a few seconds. 4. If the pump works only intermittently, worn brushes may be the cause. When replacing the pump, it is worth renewing the filter and cleaning the tank.
What is the normal fuel pressure on the E34 and why does the pressure drop after shutdown?
The reference value depends on the engine: M30 about 2.3-3 bar, M50 about 3-3.5 bar and M60 about 3.7 bar. A pressure of 1-1.5 bar points to the pump or the pressure regulator. If the pressure disappears while standing, the cause may be a leaking pump check valve, pressure regulator or injector.