2000 bmw 323i turns over and runs for a second then shuts off immediately.

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I have a 2000 bmw 323i that I've been working on for awhile now. Used to run fine but all of the sudden it stopped. It will turn over and even fire up just for a moment and immediately shut off even if you give it gas or not. I've tested the fuel pressure and I get 50 PSI then it suddenly drops after it primes. I've replaced the fuel regulator and the vaccuum hoses that go to the regulator. I've even put a check valve outside of the fuel tank just to see if I could get pressure to see if it would run. The problem is still ongoing. Im hoping somebody can shed some light on this