BMW 318/325/M3/525/535/M5 1989-1993 Repair Information

Description of Systems



For a complete description of the operation of the Motronic fuel injection system, refer to Emission Controls .


The fuel pump provides fuel to the fuel injection system at a high pressure and at a high enough flow rate to supply the demands of the fuel injectors. The pump provides enough fuel pressure that the pressure must be reduced by a fuel pressure regulator. The fuel flow provided by the pump is beyond the needs of the fuel injectors and most of the fuel flow is returned to the fuel tank.

The pump is a roller-cell design and is electrically powered.The fuel pump is mounted in the tank and can be reached through a panel in the floor of the trunk. On the top of the pump is a pressure damper and a check valve that can be replaced. Wiring for the pump and gauge unit are on the same connector. The pump can be tested separately for delivery quantity but the pressure test is for the whole fuel system.

The flow from the fuel pump is sent through a fuel filter that prevents dirt and contaminants from fouling the fuel injectors. After the fuel passes through the filter, it goes to a fuel pressure regulator that controls the pressure at the injectors. The fuel pressure regulator is controlled by the manifold vacuum and maintains the pressure differential between the manifold vacuum and fuel pressure. As the vacuum drops due to larger throttle openings, the fuel pressure is increased. The excess fuel is returned to the fuel tank.

The fuel at the correct pressure for the given moment in engine operation is queued in the fuel rail. The fuel injectors are mounted in the intake manifold and have the fuel rail mounted across the inlets. The fuel injectors are electrically controlled pintle valves. As the injector receives the signal to open from the Motronic control unit, the current flows into the solenoid winding and pulls open the pintle valve approximately 0.05mm. The fuel sprays out of the tiny opening at the front of the injector in a finely atomized mist. The extremely small droplets ensure complete combustion and efficient operation of the engine.