The reason to replace the thermostat is to eliminate any possibility the thermostat has anything to do with you problem. I don't think that is the case, highly unlikely, but the electronics do read engine heat and do adjust fuel/air mix.

Pinching the return line with the engine running increases the pressure at the engine and could account for the engine running normally. The same is true when you were doing the test at the fuel rail. It would start but if you waited until the pressure built up some it would fire and then stall. When you tried to start it right away there wasn't enough pressure to fire up the engine.

This is important---Is it correct that the original symptoms and the symptoms after the 500 good miles were exactly the same? If that is the case and the replacement fuel pump was the fix the first time, it is very logical to expect the same symptoms indicate the same solution.

Since you have replaced the pump twice, it is logical to check if the voltage to the pump is constant.

I would repeat the last test with the engine not running. If the test of pinching off the return line, with the engine not running, does not show a rapid increase in the pressure when the line is pinched, I would assume the pump is not working right.

To determine if it is poor or fluctuating voltage at the pump conectorDisconnect the connector from the fuel pump and fuel level sensor.
Make sure fuel pump voltage is 12.6 (assumes battery is good and fully charged).
Attach ammeter between pump lead and harness.
Connect jumper wire between pump and harness. Be careful not to short to ground.
Attach jumper wire to fuel pump relay socket.
operate fuel pump by removing pump relay. Connect relay socket 30 to socket terminal 87 with 14 gauge wire that has 15 amp fuse link and toggle switch if you can. Fuel pump operates when jumper is connected.
The normal fuel pump consumption is 9.6 amps. Higher than normal indicates worn fuel pump which will cause your symptoms. Lower than normal may indicate fuel line blockage.

The above is how to test the voltage to the pump how many amps it is drawing.
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I'm already replacing the fuel pump with a new one, so regardless if the voltage the pump is throwing is working or not, It's going. Now, if there was a problem that the pump wasn't receiving enough voltage or current, could this be causing the problems I am experiencing?

I'm just afraid that come Monday when I get my new pump in, I won't fix the problem! If a new pump doesn't fix it, what else could I be looking at?

Yes, I saw you ordered a new pump. Sure hope that fixes everything.

Yes, fluctuating current would result in fluctuating fuel pump performance.

That's a long shot but it would mean tracking back to the power source looking for a frayed wire or connection that is robbing current.

There is also the possibility that when Adrian installed the fuel filter he got the two bottom hoses switched. One is the supply and the other is the return.

OP had 500 miles of no problem after he replaced the filter--see recent summary.

The reason why I asked is because in one of his video, the fuel pressure is rising up extremely slowly and with the ignition off.

I noticed that, didn't know what to make of that---strange.

Hey everyone,

just checking in. I still haven't received my new fuel pump, but once I do, I will be installing it and seeing if that will fix my problem.

Quick recap: The vehicle was holding some type of pressure after installing the new fuel filter and pressure regulator, something that wasn't happening before.

When the vehicle starts, it goes to a full pressure of about 52psi, but diminishes the longer the vehicle is kept on.

IF, and this is a very scary IF, the new fuel pump doesn't fix the problem, what else am I looking at?

INPA doesn't find anything, BMW Scanner 1.4 doesn't find anything.

What else should I be preparing myself for?

I'm so scared that the new fuel pump will not fix the issue that I'm having nightmares. I've been getting by with rides to work, but this isn't sustainable. I'm just so worried....

/vent

When you are going through hell, keep on going.

If the new fuel pump does not fix the problem it will be time to start again with testing at the tank and then move forward and test at each point fuel flow could be restricted.

Until you have consistent fuel pressure at the fuel rail there is not reason to think the problem is anything other than the fuel system.

When do you expect the fuel pump to arrive?