My 2001 F4I started idling low but would always stay running. Today I took it out and it died on me, and dies every time I let off the throttle. Any ideas what it could be? Thanks!

 

check for vacuum leaks, make sure the air filter is clean. check what color the plugs are.

 

May be a silly question, but have you tried increasing the idle by turning up the throttle idle screw? If that doesn't work then clean the plugs and as suggested check air cleaner. Also check battery is getting a charge by connecting a multi meter across the battery terminals and rev the engine. You should see an increase in voltage; if not then battery is not getting a charge. You'll have to eliminate things one at a time I'm afraid. Get yourself a Haynes manual for your model. They are a great help.