OK,Newbie here be gentle; 1999 F150,2WD,5 speed Manual tranny,4.2l 6. Truck idles all over from 900rpm to 2100 rpm; Have installed following, TPS,Cam Shaft Position sensor and synchronizer,fuel filter,plug wires and plugs,iac,new air filter. IN 1st gear truck will run up to 2100 rpm and then fall to 900 and then back up. Lastly,with engine cold, started and sprayed starter fluid everywhere checking for vacum leaks; none detected. Your suggestions will be greatly appreciated or source for 2 to 3 sticks of dynamite,I'm that frustated.

 

did ya clean out the air passage into the manifold when you changed the IAC? could be carboned causing problems.. also try cleaning your throttle body

 

did you clean the maf sensor? any codes?