when sitting after 12 hours it won't start just quits until I pedal it

when sitting after 12 hours it won't start just quits until I pedal it My 2001 gas 250 5 4 super duty runs fine starts on a dime good power .but when sitting after 12 hours it won't start just quits until I pedal it then I'm good just replaced the fp idle air c v and Mas. So now what?