I have to keep my foot on gas to keep running but when i let off 99% of time it dies

SOURCE: Dies when I take my foot off the gas peddle.

Your Fuel Pump or Fuel Filter is Bad. Replace the Filter and see where you are.

SOURCE: 99 F150 5.4 V8 - Will start but then dies.

Yes your idle cotrol valve is sticking replace it and it will fix your problem

SOURCE: the car starts then dies

this may be your iac unit. go to auto zone have them hook up a scanner and read your computer codes, this will tell you where to start looking.

SOURCE: 1998 ford expedition won't stay running, i can

Start with the vacuum hoses on the motor, this will do this. the vacuum hoses are rubber hoses three quarter inch in diameter on down to real small hoses and can be located anywhere on the motor or under the hood. Look for a hose off, cracked, broken, or loose at a connector.

SOURCE: 1995 ford eco van. Very

I would change fuel filter first, but may be fuel pump??