Cranks but wont start

88 bmw 750il
when i start the car and hold the key turned it its starts for splitsecond then try to shut of. has long had o hold the key in the turnpistion it well do the same thing untill i drain the battery. then ihave to jump the battery again. some one said fuel pump and i wass toldon htis car it has two fuel pumps. so if thats the case shouldnt one ofthem still work to push fuel to the engin.

brand new battery
and half tank of gas.