Getting the air out of a fiero cooling system is a process unto it own.
First park it on a slight slope so that the front radiator is slightly lower
than the back (engine) filling tube.  Take off both radiator caps. fill the
system with coolant until it starts to come out of the radiator in front.
Put a radiator cap on the front but not the back. Keep adding coolant to the
back till it fills up to the bottom of the fill tube. Start the engine. let
it run and watch the coolant level in the back. Everytime you can't see the
coolant in the back add more coolant. Eventually after a few minutes
probably and hour, the coolant will stay in sight in the fill tube. Remember
that there is a front radiator, and a long pipe from the front of the car to
the back and the engine that all have to get the air purged. once you have
done this procedure you can reinstall the thermostat and back fill tube cap.
While you are at it turn on the heater too to purge air out of the heater
core. When I did my 86 SE I got burned a few times because that sucker gets
hot.

here is a useful link for anyone with a Fiero.
http://www.geocities.com/MotorCity/Garage/5007/fiero_osg/osg.html