My 88 V-6 is producing either a lot of pressure or vacuum build up in the fuel tank.
So much so that when I open the gas cap it releases fast.
Is the vent line that runs to the charcoal canister open or does it have a check valve somewhere by the inlet to the tank?
I tried blowing air into the line after removing it from the canister end but no air will flow through.
I can`t seem to find a diagram anywhere on the web.
Thanks

IIRC, the only valve in the evap system is the one on top of the charcoal canister. It should be open pipe from the canister to the fuel tank.

see https://www.fiero.nl/forum/Forum2/HTML/143152.html

------------------
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

Thanks theogre, I will try to see if I can find where it`s stopped up.
I didn`t know that there was an expansion tank because the line that ran down under the
car from the charcoal canister, I thought ran directly to the fuel tank.
Hopefully, I won`t have to remove the rear clip but I may have to.

There is also a check valve on the fuel sending unit in the tank. There's a ball float to prevent raw fuel from being pushed up into the evaporative emissions system. It's attached to the bottom of the sending unit flange, high above the normal fuel level, but fuel could be sloshed up that high during rapid turns, stops or starts or when parking on a steep incline with a full tank.

The charcoal canister does have a one-way valve in it, too. I've tested one and found that one of the ports will flow one direction but not the other. I don't remember which, but I believe the one coming up from the tank is the one.

In The Ogre's link, where lemonhead replaced his sending unit, we found that it's easy to transpose the fuel return line and the evap purge line due to the way TFS steel lines are bent. One must pay close attention to how they fit back because the purge and return lines are located differently on the new TFS sending units.

My '88 does that as well. I wasn't sure if that was normal because I've only had the car for a couple weeks, but when I open the gas cap it makes quite a big sucking sound and the gas tank will even oil can.

So it's not supposed to do that?

quote Originally posted by tmadia: So it's not supposed to do that?

No. Make sure all the vacuum lines are there and connected.

[This message has been edited by Gall757 (edited 11-27-2019).]

Hey i had the same thing happen to me a few weeks back.

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.

CLICK FOR FULL SIZE

1

CLICK FOR FULL SIZE

2
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).]

Float Valve. This is located on the bottom of the sending unit flange, on the vent line. It's proably not mentioned because it's NS, being a part of the total sending unit. This is the line that should go to the vapor recovery system.

It's covered by a plastic cover. Removing the plastic cover reveals the valve. Inside is a float that blocks liquid fuel from passing into the vapor recovery system. With the sending unit removed from the tank, it can be tested, first by whether or not it will rattle when shaken. Second, by turning the sending unit upside down and applying vacuum to the line, it will block air flow.

Isn't there an expansion tank in the passenger side wheel well? If so could that be plugged somehow?

quote Originally posted by tmadia: Isn't there an expansion tank in the passenger side wheel well? If so could that be plugged somehow?

Yes, and it is item #7 in the Ogre's first graphic, but only for years '87 and '88. The metal lines run under the battery.

[This message has been edited by Gall757 (edited 11-29-2019).]

87-88 Fuel Expansion Tank - as seen from underneath the car, ahead of the rear-right wheel



I've been looking into this lately, as the source of my fuel vapour leak

------------------
James
RHD '87 Fiero GT

[This message has been edited by James_GT (edited 11-29-2019).]

Anyone know what's inside that tank?

Air that smells like gasoline. Sometimes there is liquid gasoline if you fill up on a hot day.

That's it? No filters or anything that could clog up? Seems unlikely that that could be the culprit then.

Thanks for the comments.
I pulled the tank out today and probably tomorrow will try to clean out the lines.
Tank looks good and someone must have had it out in the past because the front mounting hole has been notched
and I saw a thread that showed someone had done that very thing to be able to slide the tank onto that front top bolt
without having to try to get the bolt started, which would almost seem impossible without taking the rear clip off.
I don`t think the tank has been used for a long time as the vapors smelled really old.

[This message has been edited by TXGOOD (edited 11-30-2019).]

I think line part to EVAP can has the restriction above is likely only thing might plug up.
Tank is empty 99+ % of time.
Tank is only a backup expansion tank when main is full and then gets hot or worse some fool pushes pass full when pump nozzle first shuts off.
This was to make main tank and lines to allow ~ 1 gallon more vs older Fiero tanks.
This tank and vent line(s) may have some water in it because any air breath in can/will condense any water in that air.

If the overflow tank was removed...
If replace because old one leaked etc. then maybe didn't clean the "new" one.
If "cleaned" may have done it "wrong" now plugged again. Cleaners and rust removal product can plug the small hole or make small hole smaller after done that plug up easier.
Some use tank coating for main tank that will plug the small hole here too.

I don't know if you can see the small hole. If you can try pushing a small drill or wire thru it to make sure you remove rust etc. Size is likely non critical but don't wallow out the hole either.

Quick EVAP leak test is unplug line to EVAP can then use old tire valve cut to fit the rubber hose so can easily put ~ 6 psi air in the system using a hand pump then monitor air lose. If air is gone after only a 1 minute or 2 then go to a shop w/ smoke test to quickly find it/them. Get a new Gas cap then retest might help and cheaper if that the leak.

Why Bother? Leaking gas fumes is bad enough but that lets water in and causes big problems w/ E10 fuels.

quote Originally posted by theogre: I think line part to EVAP can has the restriction above is likely only thing might plug up. Tank is empty 99+ % of time. Tank is only a backup expansion tank when main is full and then gets hot or worse some fool pushes pass full when pump nozzle first shuts off. This was to make main tank and lines to allow ~ 1 gallon more vs older Fiero tanks. This tank and vent line(s) may have some water in it because any air breath in can/will condense any water in that air. If the overflow tank was removed... If replace because old one leaked etc. then maybe didn't clean the "new" one. If "cleaned" may have done it "wrong" now plugged again. Cleaners and rust removal product can plug the small hole or make small hole smaller after done that plug up easier. Some use tank coating for main tank that will plug the small hole here too. I don't know if you can see the small hole. If you can try pushing a small drill or wire thru it to make sure you remove rust etc. Size is likely non critical but don't wallow out the hole either. Quick EVAP leak test is unplug line to EVAP can then use old tire valve cut to fit the rubber hose so can easily put ~ 6 psi air in the system using a hand pump then monitor air lose. If air is gone after only a 1 minute or 2 then go to a shop w/ smoke test to quickly find it/them. Get a new Gas cap then retest might help and cheaper if that the leak. Why Bother? Leaking gas fumes is bad enough but that lets water in and causes big problems w/ E10 fuels.

Thanks, I`m going to replace my fuel sending unit and fuel pump in a bit.
I will check all of the lines then as I do want it to operate the way GM intended.