why is this happening?

it could be a number of things....click on link below and clean maf,iac and check the tps...pull codes and repost results. a simple tune up including new fuel filter,a can of seafoam in the tank, air filter,plugs,wires, rotor cap and button may help....http://www.muscularmustangs.com/maintain.php

It doesn't have a maf. I put in a new fuel filter a week ago, did a fuel system flush about a month ago, the air filter looked fine, the plugs and wires are a year old max.
 
mine does the same thing...its just like the feul system isnt pumped up....or "primed" enough before starting it.... But like you describe if i start it up and then dont give the gas a tap itll probably die. THere are no other bad symptoms in my case to speak of either..

But i can turn it off and even after a few hours itll start right up fine...its just after a day...or days... of sitting
 
iwashmycar
mine does the same thing...its just like the feul system isnt pumped up....or "primed" enough before starting it.... But like you describe if i start it up and then dont give the gas a tap itll probably die. THere are no other bad symptoms in my case to speak of either..

But i can turn it off and even after a few hours itll start right up fine...its just after a day...or days... of sitting

+1, exactly the same for mine. no codes, doesnt really bother me.....