Gas filler door....
 

It wont stay closed on its own. If I lock the doors, it will of course. But I forget, it flops all over the place while driving...

Any ideas, if it's a spring or what?

there should be a tension spring - when you lock the doors from the remote, the flap locks too, when you lock the car from the button inside the car or when the car auto locks when you drive off, the flap does not lock - normal operation...

it is not supposed to be flapping around - check to see if some screws are loose...

Ya, bolts are tight. There is what looks like a clip/spring that should act like it would hold it closed..

It's strange because it has been fine up until a week ago. Now there's no resistance at all.

see if this part is the one you need - 51171969235

it is about $2.00

do a picture search on google

Ya, that's the one. It must have lost one side of the tabs or something.

Off to the stealership!

Lucky you - I would be happy if all I had to do was replace a $2 part :)