New Member. Fuel EVAP system and gas problems

[Deleted]

Greetings! I found this site and am now a new member. I have had my Charger for 1 ½ years. It is:

1972 Dodge Charger Hardtop
318/904/8.25
Matching numbers everything,
Originally HT8 Dark Tan Metallic (read Brown)
Color change on the 17 year-old restoration to FY1 Top (fine with me!)
Simple mods—4bbl, duals.

Here is a question and topic: Fuel EVAP system and gas problems.
I have been trying to understand this issue that I am having and have read about it in other forums, but still can’t exactly get the answer for our cars.

The story,
When I picked up the car, I noticed that the gas cap was loosely fitting, so I stopped by an Autozone and got a new cap that fit tight.
Later after some days and hot days, I heard the gas tank bang when the car was in the garage after a trip. I thought, huh? What the hell was that? I started reading about it and realized that the PO had removed the charcoal canaster and hoses, etc. during the resto and capped off the vent line up near the engine bay. This explains the PO’s use of the loosey goosy fuel cap. So, I realized that the tank needed vented and bought a vented gas cap. This has stopped the tank pressure banging problem, but a new problem has emerged….

Now, when I have full or close to full tank, the gas will start expanding in the tank and force it’s way out the filler neck (slowly as the cap is pretty tight.). This causes a gas leak and strong smell. I couple of times, I had to siphon gas out to stop this dangerous condition. Now, I have been sure and not to fill up the tank until I figure this out. I do have the charcoal canister and some hoses, and understand the system and can probably restore it and have it back in place. The bracket is even right there in the engine bay. I understand doing this, I would return to a proper non-vented cap.

So the questions…. Is restoring the system as designed really going to stop the gas expansion leak/full tank issue?
Also, in the FSM, there is a diagram of the system mentioning an “overfill limiting valve”. I have not been able to find this valve. Where exactly is this valve? I have heard it is at the liquid vapor separator, or sometimes I hear it’s supposed to be up front near the charcoal canister. I have a Feb. 15th build date, so the liquid vapor separator is in the driver’s side rear wheel well and the vent lines come off the side of tank. Perhaps the overfill limiting valve not being there or not functioning is causing me to fill the tank up too much? I can’t imagine if the EVAP system was restored that all that vaper expansion would fit in the canister. I’m worried that the expansion issue would overwhelm the vent lines and fill up the canister with liquid fuel.

I appreciate anyone’s advice, experience and thoughts. It is unsettling to have this potential gas leak when my tank is full or even near full. It’s hard to tell how much to fill up the tank to avoid the issue.

Thanks, Brian

[DynoDave]

Welcome aboard topbanana72!



I have no experience with this. My GUESS (and that's all it is) would be that yes, with a near full tank, the vent will help.



Lets use this cutaway as an example.

My GUESS is that there is always going to be some trapped fumes in the top of the tank. See how the bottom of the filler pipe would be below liquid fuel level when completely full? Especially if you "top it off"? That would leave trapped fumes at the top of the tank. Those fumes would NOT be able to escape up the filler neck to your vented cap when the bottom of the pipe is submerged. As those fumes and fuel expand, and create pressure, that would back liquid fuel up the fill tube.

A proper vent from the top of the tank would relieve this pressure.

Again, just a theory. Take it for what it's worth. Other opinions are welcome!

[Deleted]

Thanks Dyno! When you say: "Those fumes would NOT be able to escape up the filler neck to your vented cap when the bottom of the pipe is submerged. As those fumes and fuel expand, and create pressure, that would back liquid fuel up the fill tube." I think that is exactly what's happening. Also a non vented cap would hold back that pressure and not allow the filler tube to keep filling from the expansion, especially if the upper vent is available. But, because it wasn't at first, I got the pressure bang. I'll have to inspect and test the vent system. I think the top tank area has the expansion space and four vent lines from each top corner - running together for exit at the upper-side of the tank and up into the above liquid-vapor separator as designed. If I uncap that vent line up by the engine and restore the charcoal canister, maybe it will all work!

Any idea about this "overfill limiting valve" ?

[quapmaster]

There should be 4 vent tubes coming out of the front or the side of the tank that go to a vapor chamber, which then allows the fumes to condense back into liquid and drip back into the tank. In the wheel well on 71-early 72 and in front of the tank mid 72-later.

[Deleted]

Copy that quapmaster. I have that, the early 72.

[quapmaster]

No clue on the overfill valve.

Do you have the giant expansion chamber in the wheel well?

Yes, you could re-plumb the charcoal canister, or go with the 71 design where the engine compartment vent tube (from the rear expansion tank) just vents to the breather. You'd need to get a 2 or 3 nipple breather.

nebula.wsimg.com/obj/RDIwQ0ZCMDI0QTVCMTkzNkI1MDg6MjBiZWQ3NDYyY2Y3ZjM0YThkYjZkMWFlZTIyNGMzYTc6Ojo6OjA=

[quapmaster]

[DynoDave]

I don't "know" about the overfill protection valve, but again, can make what I think is a pretty accurate educated guess. Opinions on the accuracy of my guesses do vary.

The charcoal canister is designed to capture fuel vapors only, and feed them to the carb under the right conditions, to be burned in the engine. No escaping hydrocarbons, for the environment or your garage. It will not be effective if soaked in liquid fuel. Thus, that overfill protection valve is in the vent line, after the separator, but before the charcoal canister. Not being OCD about filling your tank to a round dollar amount, or feeling obliged to squeeze in every drop possible also helps avoid this, valve or not. So my guess is this valve is there to protect the charcoal canister against tank overfill, not to prevent fuel tank overfilling.

I've never seen one of these valves.

And I'd forgotten about the 3 nipple breather. My '70 Duster has one. Thanks for the reminder Quapmaster.

[setauketjeff]

Also remember that the older 440's also have a vapor line coming back from the gas filter which also goes back to the return. Dodge ended pressurized fuel systems mid 1971 production (I.E. My first week of 1971 production Super Bee uses the pressurized system).

[Deleted]

Thanks guys...


I do have the giant thing in the wheel well. It would be the liquid-vapor separator. I have not seen it, but that is where the vent lines point. There is supposed to be a panel in the wheel well you can remove to inspect it. I know this, because, curiously, Classic Industries has a reproduction panel. Just as curiously, nobody repops the charcoal canister or the liquid-vapor separator, so I hope mine is in good shape.


I think I am going to attempt to restore the charcoal canister system as designed. The 71 system was a stop-gap design that caused hot starting issues, according to what I read. The 72 and up charcoal canister concept was apparently a breakthrough and the best from then on.



Regarding the overfill limiting valve, it indeed is suppose to help prevent overfilling the tank. It is closed during tank fill up as the head of fuel reaches the top of the tank. Afterwards, during thermal expansion, the expansion and vapor pressures are enough to open it to allow the vapors to go to the charcoal canister. I think I will keep looking for this elusive valve. I have a feeling every '72 Mopar I see at the shows this year, I am going to be looking or this thing LOL.

BTW, totally with not forcing too much fuel in the tanks, especially when the filler neck is so low.