How far off is it? Let guess, 1/4 tank? The fuel system transfers fuel from the right side to the left side via a siphoning system. Both sides have a sending unitbthat measures fuel by averaging. If the fuel on the right is not being transferred to the left then you can have fuel remaining on the right side and be dry on the left where the fuel pump is located. When you replaced the pump you may have created a leak that is breaking the siphon effect. Can’t tell you how to solve this other than open up both sides and check the connections. It possible that you right side filter unit is bad. Since your vehicle and the donor are older it’s possible that all parts are worn out.

It’s probably the balance issue between the two sides unless you possibly jammed the float somehow and it’s not swinging freely. Have you run out of gas? I would try opening up the pump side and repositioning everything. Also you can verify if there is actually fuel on the pump side when it reads 1/4.

Take my word on this. There is a float assembly on both sides. If you ran out of gas at 1/4 tank it’s because there was no fuel on the pump side (drivers side). There’s another assembly on the left where the fuel filter goes. That side was registering 1/4 tank because there was fuel on that side that didn’t transfer over to the drivers side. Fuel is sucked up by the pump on the drivers side. Engine ran out; no fuel for the pump to suck.

I don’t know if a way to reset it. It is most likely a fault in the system that balances the fuel from the right side to the left side. you can try removing the pump and reinstalling just in case the float on that side was hung up, but I doubt it. You ran out at 1/4 most likely because the right side of the tank had fuel in it and was registering it via the float on that side. The drivers side was probably empty because the system that is supposed to transfer fuel right to left is faulty. The only way to verify this is to open up the left side when it runs out of gas and visually check to see if there is fuel on that side.

The only way you can definitively verify what’s happening is to run the tank down till it reads 1/4. Then pump out a gallon or so. Start the vehicle and see if it quits. If it does, pull the pump and check the fuel level. If it’s dry then you can open the filter side and verify whether or not there is fuel on that side. I know that’s a pain in the butt, but you can thank BMW for designing a vehicle with a split tank and two sending units. If there was only one sending unit it would simplify things immensely. Clean the connections as ItsMeWayne suggested.