You have a busted transfer pump. The transfer pump use siphoning action to move gasoline from the left side of the tank to the right side of the tank. Siphoning is when the fuel pump is running, but with a bad transfer pump the siphon doesn't occur. Put the pump back in and dump a gallon or two of gas into the right side. If the engine start afterward, the fuel pump is o.k. You can fix the transfer pump (Better be sitting down when they tell you how much that stupid transfer pump is gonna cost) or just remember to fill up the tank when the gauge reads half full. The float for the fuel gauge is on the left side, that is why the gauge read half full since the left side is half empty.

Edit:

The fuel rail pressure should be 3.5 Bars not PSI. If you have less than 10 PSI the engine is not going to start.

http://http://parts.bmwofsouthatlant...ssembly=259479

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