fuel delivery problem

I believe I have narrowed down an idle and drivabilty problem to an overly rich condition which only affects performance when the throttle blade is closed.this also explains why you have to slightly open the throttle before engine will start.my question is besides a faulty injector or injectors,what else could be causing too much fuel to be delivered without there being any trouble codes set?engine has poor idle and dies when taking foot off pedal and coming to a stop,but otherwise runs perfectly.thank you for any help with this.