My 1988 LFX F40 build. (Page 45/68)
Rickady88GT JUL 29, 11:50 PM
The canister is sized according to or relative to the volume of the fuel tank. The larger the fuel tank, the larger the vapor canister.
It is about capturing the gasoline vapor (as the ambient temperature and road/parking lot temperature increases, causing the gas tank to heat up and vaporize the gas) and holding it till you start the engine. The PCM, ECM then allows engine vacuum to purge the canister of raw gasoline vapor and NOT allow it to vent to the atmosphere.
Your little canister is not properly sized for the fuel tank of the Fiero. Thus, raw vapor will vent to the atmosphere.
Joseph Upson JUL 30, 07:22 AM

quote
Originally posted by Daryl M:

I'm trying to wrap my head around what is going on in this system. It is a closed system, so flow can't be too much. The whole purpose of the system, as I understand it, is to allow for expansion and contraction due to temperature changes and to prevent a vacuum to form as fuel is used in a closed system. That is about 4 or 5 liters of air displaced every 30 minutes . If all it is doing is absorbing fuel vapors from air, that isn't much fuel, or am I missing something?



As others have chimed in and pointed out, a lot of expansion takes place in the tank. Another observation I made that clued me into the vent line being plugged, was the occasional crumple sound the tank would make as those gases expanded and were released upon removal of the cap and an important other point that was pointed out above, is canister saturation. This is general information that some have learned the hard way; When the gas pump handle automatically shuts off indicating the tank is full on modern cars, STOP filling. Trying to top off the tank further can result in fuel saturating and ruining the canister, which more often than not, is located on top of the fuel tank, which can be a very expensive repair for those who need, or want the engine light off after startup. The EVAP solenoid is a commonly failing part on modern GM cars and when replacing it doesn't clear an EVAP code, it usually means there's trouble at the canister.

Look up a simple EVAP diagram on how the system works for more insight.

[This message has been edited by Joseph Upson (edited 07-30-2020).]

ChuckR JUL 30, 07:53 AM
Here is a good video that shows all the components, and how they work Take a look at that canister and how much active charcoal is in here. I have this link set to start at this point, but this guy goes into detail throughout the whole system. Hope this helps to get your head around it. I would find a place to add in the canister from the donor car, or possibly replace the Fiero expansion tank with the modern canister. The Fiero expansion tank is doing only part of the job these charcoal canisters are designed to do in more modern systems.

https://youtu.be/EA3fm_UackQ?t=618
Daryl M JUL 30, 04:07 PM
So the take-away is that a charcoal canister that is too small will not have the charcoal surface capacity to absorb sufficient vapors, resulting in vapors being released into the atmosphere?
Rickady88GT JUL 30, 04:50 PM

quote
Originally posted by Daryl M:

So the take-away is that a charcoal canister that is too small will not have the charcoal surface capacity to absorb sufficient vapors, resulting in vapors being released into the atmosphere?



Yes.
It is also true that raw fuel vapor is the most harmful to the atmosphere. Nothing that comes out of the tailpipe is worse except for the unburned fuel. But I am not getting all tree hugger on you, any raw fuel vented out can smell awful in a garage and may even be hazardous.

[This message has been edited by Rickady88GT (edited 07-30-2020).]

Daryl M JUL 30, 05:09 PM
So the take-away is that a charcoal canister that is too small will not have the charcoal surface capacity to absorb sufficient vapors, resulting in vapors being released into the atmosphere?
Daryl M JUL 31, 11:23 PM
Ok, so the system is closed. How is the pressure differential , caused by temperature changes, equalized when the vehicle is turned off?
pmbrunelle JUL 31, 11:42 PM
A fuel tank isn't a closed system; if it were closed, you wouldn't need a charcoal canister. No vapours could escape.

To avoid pressure buildup, a fuel tank is vented to the atmosphere. Naturally, this causes fuel vapours to escape via the vent. In modern cars, a charcoal canister is placed inline with the vent line, to adsorb the fuel vapours. The air continues to pass freely through the canister, equalizing the pressure with the outside.

At some time when the engine is running, the canister will be purged of fuel it had adsorbed. The engine will suck gasoline-laden air from the canister, removing gasoline from the charcoal.

Now that the charcoal has been purged of gasoline, it is ready to again adsorb fuel vapour the next time the car is parked.
pmbrunelle JUL 31, 11:53 PM
Also, it's adsorb, not absorb.

The gasoline is adsorbed onto the surface of the charcoal granules.

The gasoline is not absorbed inside the bulk of the granules.
Daryl M AUG 01, 10:48 AM

quote
Originally posted by pmbrunelle:

Also, it's adsorb, not absorb.

The gasoline is adsorbed onto the surface of the charcoal granules.

The gasoline is not absorbed inside the bulk of the granules.


Ok, but exactly where is the vent to the atmosphere? Here is my issue. The original canister has a "soft bottom" . I assume that is where it vents through. On the original car configuration, the purge seems to be controlled by engine vacuum with a diaphragm on the top of the canister. Also, in the original configurations, the tank vent is 3/4" and goes directly from the tank to the top of the filler neck. This means that vapors, during filling of the tank, exit to the atmosphere without being absorbed because the canister is not in play when fueling. The swap engine I am using is a 2013 impala 3.6l. On that car, the canister purge is controlled by the ecm and an electrically operated purge valve. The impala canister does not have a vent. I assume the vent is elsewhere in that system. It seems to me that neither the Fiero canister nor the impala canister will operate properly on my swapped car unless I modify the canister or ad a vent somewhere.