94 explorer engine layout

I recently got a 94 ford explorer xlt and it came with out an owner's manual. well today i was looking under the hood and I noticed that there was a tube comming from somewhere in the engine compartment and it went directly into the radiator and on the tube it had like a little plastic knob on it that could be twisted off. I was wondering what is it for?

and also, where is the fuel shut off switch I looked all under the dash and under the passanger foot rest and i could 'nt find anything that would resemble like switch or a botton. where else could it be?
Thanks. EA