Do this (actually if it smells that bad you probably don't need to wait). I would not touch the sender if you don't find a problem. I've seen a bunch of guys posting that new senders not working and either gasket or ring are undersized causing leaks.
Possibility. I'd fill the tank clear full and let it set in a cool area for 1/2 day or so and then look around under there for 'wet spots'. It's always possible something rusted through, and finally failed.
