BMW E46 diesel: transfer pump and rail pressure
What is the E46 diesel transfer pump and where is it?
The in-tank pump feeds fuel to the transfer pump, which raises the pressure to about 3.5-4 bar. After that the high-pressure pump produces about 300-1350 bar depending on load. The transfer pump is on the driver's side under the seat and near the footwell.
How do you check the transfer pump's operation?
Turn the ignition key to position 2 and listen for the pump's hum. If the car does not start, the pump switches off after about 60 seconds. Measuring the transfer pressure is more reliable: under 3 bar points to a pump fault. If the pump does not run even though the voltage is fine, also check the ground wire and the pump relay.
Why does the E46 diesel start badly and the rail pressure not rise?
Causes include a leaking injector (the fault may be in a single injector), a broken crankshaft sensor, loose high-pressure lines or a faulty pressure regulator. Follow the rail pressure in live data during cranking: if it does not rise, check the sensors and connections first.
Why does the diesel go into limp mode at full throttle with a rail pressure code?
The cause may be a fault in the high-pressure pump's pressure control valve, a loose high-pressure line or an injector that leaks into the return side at higher pressures. Check these before replacing the pressure sensor or regulator.