i have a 1990 f150 with the 5.0 lately it has had a fluxuating idle on start ups since it started getting cold out. i figured its something to do with a vacumm line somewhere. well monday the truck all of a sudden started bucking and missing when you put it into drive and go to pull out. but once you start driving its fine. but then once you slow down, or go to park it starts bucking and missing and on occasion dies. i just replaced the plugs, wires and cap and rotor today and its still doing it. could it be the idle air control valve or the throttle position sensor? the previous owner said it did it a long time ago and he replaced the idle sensor, but i cant find anything about the idle sensor, only thing i could think of thats close would be the iacv. any help is greatly appreciated. trying to fix it asap

 

it is most likely you iac valve,i had a 93 bronco with a 5.0l and the same symptoms and i tested the tps sensor and it was ok so i took the iac valve off and it was carboned up ,so i took the valve apart and used i solvent free penitrating fluid to clean it up then coated the internals and put it together and now it idles smooth and the fast idle even works. i hope this helps:wave

 

Most likely a blown plenum gasket. With the engine running, spray carb cleaner just behind the distributor where the upper intake bolts to the lower. If the gasket is blown, the idle will smooth out. Common problem on these. I recommend going to the dealer and getting the gasket for a 94 F150 5.0 because these were made of some kind of plastic and don't blow out.

 

well i looked on some other forums before i got an answer hear and saw that the iacv is a common problem. so i pulled it today and cleaned it realy good and its running a ton better now. next on the list is to fix the intake mani gaskets. tired of the fumes when the heat comes on. thanks for the help also