1991 BMW 325 car will only start in temperatures 0ver 35 de

First, is your check engine light illuminated??? If so, this is the place to start.

As for your cold whether hard/no-start, it's hard to give you an accurate diagnosis without doing some tests.

I've seen a few things cause this problem in extremely cold whether, but most often it's either moister (water) in your gas or your fuel pressure regulator is leaking and bleeding fuel back to the tank after you shut it off. So when you go to restart, it cranks, and cranks, and cranks, trying to get the fuel back up from the tank to the injectors. I'm not sure why this happens so often only when it's extremely cold outside.

But whenever you have a starting issue, always check to see if you have spark first (including timing), then fuel pressure and that your injectors are firing. Then you'd usually (old-school) check to see if your engine has compression. But I like to quickly check that the computer is recieving an input signal from the crankshaft position sensor. Because these are often a problem, and your engine certainly won't start without one.

Then I'd do a compression check.