5.7 motor idles at 1800 rpm. will not drop down til warm and restart. on 1995 Chevrolet 3500 Pickup

then it takes about 20 seconds to idle down to 800 rpm. have replaced all components with no luck

Asked by for the 1995 Chevrolet 3500 Pickup
Vacuum leak or IAC, air is getting inside of the intake from somewhere it isn't supposed to!! Or to much form where it is supposed to.
Good scanner and operator can find this problem. Not a parts store employee with a code reader.