If I recall there is a small nut on the bottom. If you remove that the case will slide off. There are very thin wires inside. Three maybe? I forget. But its possible they are damaged, corroded or dirty. If it sat for this long I would recommend taking a close of look possible at all parts or you will just be chasing a vicious circle. Somewhere on here is the proper ohm readings for the sending unit. So with those you can read it as you manipulate it out of the car. If its fine you can probably put it back and never have to question it again.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car