need help with fuel pump

Hold on. This a 840 or 850?

850 has two pumps, one for each cylinder bank.

there are numerous things that can cause the pumps not to start on the v12.
1. Hidden fuse is broken. 2" from the battery under some heatshrink is an 80A fuse that breaks due to metal fatigue. This will result is now power to the pumps.
2. Check fuses in the fusebox first of all, look at the list on the fuse box cover, should be between fuse 20 and 25 if memory serves me right. Two fuses, one for each pump. These are powered by relays, which power is controlled by the hidden fuse.
3. crank shaft sensors. These are located in the 2 and 5 o'clock postion on the small flywheel at the front of the engine, the one that looks like a big gear. These sensors gives pulses to the engine DME, which then triggers the relays powering the fuel pumps. No pulses, no fuel.

then there is always a chance the relays have seized or failed.

On the v12 when you crank, it gets no fuel before the DME senses the pulses from the crank sensors, then it starts the pumps, hence the somewhat unique startup sound of the V12.
putting the key in position 2 will not provide power to the pumps as on most other cars.

I would locate the fuel pump fuses, pull them, and measure the voltage while cranking, if you have power to both pumps, then I would start to look at the pumps, if not, the list is above.

not questioning your mechanic friend, but these cars are somewhat special, and normal troubleshooting does not always apply.

if you have a v8, this will not apply and I cant help you.

Even if it is a V8 - you have helped as the V8's don't run the pump until the crankshaft sensor pulses are seen by the DME!

On the V8 the pump is running for a short time (1-2 Seconds) when you switch the ignition to position 2.
It can be clearly heard when the passenger side rear seat is removed.
The pump is a combination of a pump and a fuel level sensor in a plastic frame.
The pump is kept in place by some rubber mountings. I have not dared to test how to remove them as I don't know how brittle the frame is after 17 years but it looks doable.
Unfortunately BMW doesn't sell the pump alone but there is a Bosch fuel pump which according to Bosch fits.
It has the Bosch number 0580314069 and costs around 200 Euros.

Cheers,
Steve

Correction: I found the pump for as low as 166 Euros in Germany. LINK

It's certainly worth a go seeing the saving you can make. The 540i and 740i V8's obviously have the same fuel-demand requirements as the 840ci - whether is physically fits is another question.

For that price is the MAPCO definitely worth a test. It says 3.5 bar which is exactly what the fuel line should have.
Good luck!