Hello..i have a 99 F150 with a 4.6. It is not my daily driver and I have not driven it in about a three weeks. I started it Saturday to run a few errands and the truck fired right up but only ran for 2-3 seconds. Fired it up again and the same result. It would run if I kept on the gas but would die as soon as I let off the pedal. After running with my foot on the pedal for maybe 45 seconds the truck operated normally. Any suggestions on where to start? Maybe IAC? Any other culprits?

 

You need to check the Throttle position sensor. you can replace for about 40 bucks. I changed mine last year

 

I would clean or replace the IAC valve.