1990 325i Convertible- not starting

When it first occurred, a mechanic friend replaced cap and rotor. Did not start. Next, changed relays, it started and I drove it home. Two days later, started it, drove 2 miles or so, parked, and then it did not start again. No fuel pumping. Changed fuel pump in tank. The replacement pump was from junkyard, and worked fine when tested outside of car. When put in place, car momentarily ran, then stopped again. Fuel pump not working again. Mechanic friend baffled, thinks it may be a wiring problem/short. Please help! Could it be the second fuel pump? Need direction!