Weak fuel tank pre-pump? Fuel pressure problems?

[Legoman]

Problem: Engine stalled and seemingly ran out of fuel with gauge showing ~1/4 full, OBC range saying 115km to go to empty and no red light on fuel gauge yet. 9 litres from an auxilliary jerry can was enough to restart the engine and get me to the nearest petrol station, whereupon a further 20 litres was added. Car then seemed to be running alright for a while but then started stumbling when being asked to accelerate or respond to an opening throttle pedal. I stopped at another petrol station at this point and added a further 40 litres to take the needle up to ~3/4 full and got the car home without further issue, but it was still being reluctant to accelerate enthusiastically enough to an opening throttle.

Ambient temp is only ~24°C

Car is 26 years old and has never had fuel pump problems before and fuel pumps have never been replaced. It feels to me like a fuel pressure problem rather than ignition or failing lambda based. Both lambdas have been replaced in recent history and many engine gasket/vacuum leaks have been repaired and sorted in recent history too including cam-box cover gaskets twice.

Is there an electric fuel lift/pre-pump in the E32 tank as with other model BMWs? Are these known to go weak or lose pressure after 237,000km of use? Is this a reasonable place to start testing/looking for the source of weak fuel pressure? Are there other fuel pumps elsewhere in the supply line before the injector rail pump to also be looking or is the in-tank pre-pump the most likely culprit? Could a blocked fuel filter be the cause? Where are they?

Would any of these symptoms be likely to be reported as a fault in the DME?

Any advice would be greatly appreciated.

[shogun]

There is no pre-pump, the 740 has one single pump and 2 fuel filters https://www.realoem.com/bmw/enUS/par...MW-740iL&mg=16
Last years I changed my 2 fuel pump, the 750 has 2 fuel pumps, they were 29 years old and got weaker and weaker, 1 day one of the 2 suddenly stopped as it was blocked.
As far as I know the fuel pumps function = pressure and conveying capacity you cannot read out in the DME.
You can either change the complete pump = expensive, or buy an aftermarket pump and install it in the housing. Also check the pump pre-filter which is located at the bottom of the fuel pump and that sucks all the dirt in the tank, especially when the fuel tank is at low level. The pre-filter can be cleaned.
As your pump is 26 years old, it is most probably time to replace it, also the fuel filters. Fuel pump replacement: basically same as on the 750 which has 2 pumps, yours has 1 pump, the cpl. unit is clipped in http://bmwe32.masscom.net/sean750/fu...t/Fuelpump.htm
example of installation of an aftermarket pump , here from E34, same procedure https://www.bimmerforums.com/forum/s...eplacement-DIY
Fuel filters are above right rear axle http://bmwe32.masscom.net/sean750/fu...eplacement.htm

[Legoman]

There is no pre-pump

Is this not an electric lift pump inside the tank as part and parcel of the level sender unit? Surely this is not the only pump before the injector pump is it?

[shogun]

Yes, SIR, 1 pump only, that is the fuel pump inside the tank = part of the fuel sending unit assy and there is no other pump in between the tank and the fuel injectors. The other part inside is the fuel gauge level sender.
This fuel pump produces enough or more pressure for the fuel injectors, that even there is a fuel pressure regulator in between in the engine bay. And the excess fuel pumped into the engine bay goes back into the tank this route from the fuel pressure regulator https://www.realoem.com/bmw/enUS/sho...diagId=16_0802

The so-called fuel sending unit is just to send the fuel level info to the cluster http://bmwe32.masscom.net/johan/fuel...el_sender.html

M30 and M60 have one pump, M70 has 2 pumps = for each bank 1 pump, on the E38 750 they even skipped the 2nd pump, there the M73 engine runs with one pump.

[Legoman]

Car is booked in to workshop next Tuesday. Gonna go through and check the gauze strainer, current draw on the pump motor, both filters and the regulator in the engine bay. Apparently we can source just the plug-in electric motor without having to buy the complete pump assembly from BMW here, so shouldn't be too heart breakingly expensive if indeed the pump motor is the cause of the problem.