Fiero and many other GM vehicle use the Cap on the overflow tank as Vent for normal operation and as overflow path when the car is "boiling over."
Many others have a separate vent for same reasons like black tube at the top in above pic.
Fiero and a lot of others use an "overflow tank."
It does 2 things...
1. Stops spilling coolant when people over fill the radiator until ~ 1980 when most have them. Most "Old School" vehicles did not have anything to tell you the rad is Full like now but Before O-tank the Radiators would puke the extra coolant on the ground and worse leaving Poison Ethylene Glycol coolant in puddles.
2. Purge all Air out of the cooling system. See my Cave,
Coolant Fill under "burping."
In short: System get hot force air in the system out then cools off sucking coolant in. If you fill the system right and it not leaking anywhere even Fiero will self purge in only a few heat cycles.
You can move and/or change the O-tank but watch out for Volume it can take. Fiero O-tank is much larger from Total Empty to Add or Full marks "hidden" on side toward HL assembly. look at Jeggs, Summit, etc.
Most pressured aux/surge tank system still use an "overflow tank" to do same thing, purge
All air out of the system.
Is mostly used on a vehicle where the radiator in low and very hard to get at, even worse the Fiero radiator setup, and they don't "radiator caps" in the system but the aux tank.
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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)
The Ogre's Fiero Cave