When you start it up don't completely release the key from the crank position, let off just enough to stop the starter, if the engine continues to run the ignition switch is defective, a common problem in this year. This is not the lock cylinder but the electrical switch located down at the bottom top of the steering column that the ignition key cylinder actuates via a long steel rod.
I found that the idle air control valve was stuck. I took it off and cleaned it with carb cleaner and used a pick to open and close piston to loosten it up. put it back on and truck ran fine. thanks for the feedback, even though i did not need it this time.
It is not just the valve that causes problems, it is sludge from fuel blow back in the upper intake as well. Do the following to completely clean the system out. Also you should NOT have used carb spray, it destroys the anti stick Teflon coating inside the valve. Here is the procedure.
Buy a can of throttle valve cleaner (it is made by CRC) from the local auto parts store, spray it into the air intake through the throttle body of the engine until you have used about half the can, you will need to hold the idle up while you do this, it will try to stall, after this shut the engine down and disconnect the Negative (-) battery cable for 5 minutes
MY EXPERIENCE? 25 years as a Ford dealer technican.
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If it shuts down immediately upon releasing the key, I would suspect the ignition switch. You could test my theory by running a hot jumper to the ignition coils (red wire with green stripe). If keeping that wire hot allows the engine to keep running, replace the ignition switch.
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