You may have an air leak into the fuel system, that allows the fuel to return to the tank. A full tank could have enough head to keep the air out. Parking with the nose high helps the fuel run back to the tank. I would guess that parking with the nose down would allow use of all the fuel, and nose down with a full tank might show where the air leak is, as fuel should leak out there as well.

When my neighbor owned my car, he had it towed away three times in a week because it wouldn't start. He parked nose up in his driveway and the shop was on flat ground so they had trouble duplicating the problem. I think they fixed it, but he soon sold it to me and I have a flat drive, so maybe that leaking O-ring I recently fixed on top of the lift pump was the problem.

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'97 E 300 D