Most "Dumb" EVAP Vent line are "open to atmosphere" all the time, not just Fiero system, w/ air "Filtered" by EVAP canister.
So Tank(s) should never see pressure or vacuum you can see/hear air moving a lot.
Sometimes If tank sees a big Temperature swings and drive a lot in a day then you might hear air move when you pull in to get gas but Not Normal for most Fiero and other GM owners w/ "Dumb" EVAP.
Many "Non vented" Gas Caps maybe a backup that "fail" so tank won't fail when vent is block causing high pressure or vacuum. In normal operation, they are a non vent cap and will hold some pressure/vac to force vent thru the EVAP can.
2 vac lines are Purge and Control.
Control port will never have air flow in or out to the canister.
Purge port opens when Control port see vacuum. If you blow into this when valve is closed then:
1. Many units may have a very small orifice there as a "Constant Purge" opening. I don't have time to check if Fiero uses this or not.
2. Maybe the Control valve will act as 1way but isn't made for this "Test."
GM have never mention a "1way" or "Float" valve in a tank vent line in any docs I've read in 40+ years.
Does have a "Metering"/orifice somewhere that is very small diameter. That hole is often blocked completely or act as 1way depending on exact crap in the line at that point.
87 and up is in the line between overflow tank and canister, near/at the tank.
Older is same but is near/at the main tank.
Pinched/crushed soft lines or hard line may do same.
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Origial BW from 1
Fiero 87 FSM. 2
Alldata. Then both edit/add by me to make easier to "read."
When the Control Valve is Closed, Only air and fumes will move to or from the Tank and Bowl trying to get air in or out from the bottom vent."Float" valve?
Same thing as "Shop Vac" w/ a plastic part inside the filter to stop blower when tank is full of "water." Just a lot smaller for this use.
Note: When you use a Shop Vac to suck wet crap... First check the filter(s) type. Many are Not for Wet use and will kill them and may block airflow. Standard paper, bags, and many reusable filters that look like big air filters for cars are not for Wet use.
Full text see EVAPORATIVE EMISSION CONTROL SYSTEM (EECS) in 6E Driveablity and Emisssions - General Info in most GM FSM or find exact text of a year and engine of a given model. or maybe get alldatadiy.com
[This message has been edited by theogre (edited 11-28-2019).]