why does my truck shut off when idleing

May be a vacuum leak a bad idle air control or a bad fuel pressure regulator. Since it won't idle it will be hard to check the idle vacuum. Start with the fuel pressure regulator. You will need a fuel pressure tester and a vacuum gun. Hook up the fuel pressure tester and turn the key on. Pressure should be around 60ish PSI, apply 15-20 inches of vaccum to the pressure regulator. The regulator should hold that vacuum till you let it off, and the fuel pressure should drop to 40-45 PSI. If it doesn't drop then the regulator is flooding the engine at idle. Replace it. If that tests good then pull out the idle air control and give it a good cleaning with carb cleaner and check for movement, reinstall and check how it idles. If it still idles bad then get a vacuum diagram and one by one remove the vacuum lines from the manifold, plug the hole with your finger, and have a friend start the truck. STAY CLEAR OF MOVING PARTS. Wear short sleeves no baggy cloths ties or necklaces tie up long hair if applicable. If one of them being removed allows the truck to idle then find the bad spot in the line or bad part and repair it. If neither of these work then the problem is quite possible a leak at the manifold somewhere or a burnt valve an injector stuck open and the list guess on. Try these easier steps first and let me know how it goes.