2001 ford f150 supercrew v8 2wd

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i need help!!! truck runs fine with a full tank of gas... once i get down to almost a 1/4 of a tank, it doesnt want to run unless you hold your foot on the gas. once you get it down the road about 1 or 2 miles, it will maintain on its own like it should but the rpm's will stay at .5 instead of 1 (which is where it usually is) it doens matter if the truck is cold or not, it does this. once you turn it off, you have to go throught it all over again.i dont understand why it only acts this way when it's down to a 1/4 of a tank!!!!