Type your answer here... It could possibly be the ignition switch.
check ignition
lack of fuel
A Ford 1999 Expedition engine is probably not seized if it will start and idle but will then not stay running. This could be due to a clogged fuel system, faulty injectors, faulty spark plugs or a wiring problem, or even a faulty distributor.
we have a ford escort lx and it wont start without the gas throttle half way then it starts but is very rough but wont idle by itself
you can but it wont stay running if the battery is dead
dirty carb bowl or water in the carb
have a pontiac 6000 2.5 motor and it wont stay running
bad alternator, dead battery.
Check the idle sensor if you have one.
The Optispark distributor is notorious for causing this problem.
Check the fuel pressure on the Fiero to start with. It sounds like it could be starving for fuel.
Start by checking your fuel filter.