won't idle on 1999 Ford Expedition

Won't idle after it gets warm.Will crank but won't keep running.

Asked by for the 1999 Ford Expedition
Is the Check Engine Light on? If so codes will be stored in the computer of your car and these codes will indicate where the problem most likely lye, (a scan tool will be needed to retrieve the codes using the diagnostic socket under the dash). Vacuum leaks or idle speed control motor are the most likely problem or something that fails because it absorbs engine heat.
yes check engine light is on,it will crank and if i let off the gas it dies.
The check engine light is continually on while the engine is running?

Like Patrick said, there will be codes to pull if this is the case. Have a diagnosis performed.
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To supplement Patrick - if there is not a check engine light on and there are no codes, a good fuel pressure and volume test should be performed. I have seen several of these trucks that have fuel pump issues. Low fuel pressure usually does not set any codes on this one.