gas tank vent?

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billytuffnuts

billytuffnuts

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Have a 72 demon that was originally a /6 225 with the charcoal canister in the engine bay. I put in a 360 and removed the canister but I'm not sure what to do with the line that ran from the gas tank to the charcoal canister.

As it sits the line is still in place running up to the engine bay. I want to remove it completely, but what should I to do vent the gas tank? Just make the line shorter and leave it uncapped or what?
 
Do you still have the canister? The best thing would be to keep the canister so the tank can vent fumes through the charcoal. But if not, then you can make your own vent. I put in a new tank, the one with one vent line. I bent my own vent from 1/16th ID tube. Important thing to do is to loop the vent line higher than the filler neck (gas cap). I drilled a small hole, place a rubber grommet in it and ran the tubing into the trunk. I ran the tube up the inside wheel well, into the C pillar, and back down and out the rubber plug that empties inside the rear frame rail. I placed a small filter from a mower on the end of the line using clamp and a small piece of rubber line. I fed the line down inside the frame to a place just in front of the rear spring front hanger. Easy, inexpensive, and effective.
 
I dont really want to buy a new gas cap as I already have a nice body color painted one installed.

Demon 408,

I'm a little confused on what you mean by it having to be higher than the filler neck...

Does where the vent line 'end' have to be higher or does it simply need to reach that height at some point and then could end anywhere else?
 
billytuffnuts said:
I dont really want to buy a new gas cap as I already have a nice body color painted one installed.

Demon 408,

I'm a little confused on what you mean by it having to be higher than the filler neck...

Does where the vent line 'end' have to be higher or does it simply need to reach that height at some point and then could end anywhere else?
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It must make a loop higher than filler neck, but like mine the vent end (with the small mower filter) is down through the rear frame near the front spring hanger. Sorry for late reply.
 
The loop makes sense. I didn't put one, just ran the vent tube so it was up to the top of the tank and it leaked when I filled it.
 
Take your cap off. Does the inner, that seals, have a hole? If not, just drill the inner- vented now. It's not on the outside of the cap.
 
Demon 408 said:
Yes, that it the reason. If you look at the older tank vents they all loop above the hieght of the gas cap (fill point).
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I wonder if mine might be like that. I haven't really had a chance to get underneath the car and trace the line back all the way to the gas tank but as of now the charcoal canister is removed and the metal line is basically just sitting there unhooked and has been like that for some time. I haven't had any issues of gas pouring out. Ever since I did get my gas gauge working last summer I've noticed I've been getting terrible mileage (now that I can actually see how fast the gauge moves to E).

Wasn't sure if it was the new sending unit or if it had to do with that vent line which is what triggered this question.
 
Would it be possible for some of you to post sone pictures?
 
KP said:
Would it be possible for some of you to post sone pictures?
Click to expand...

Yup got a few of the gas cap (is this a vented cap?), filler neck, and the vent line as it sits in the upper right hand corner of the engine bay. Anything else you need to see?

gascap 001.jpg


gascap 002.jpg


gascap.jpg
 
billytuffnuts said:
...I've noticed I've been getting terrible mileage (now that I can actually see how fast the gauge moves to E). Wasn't sure if it was the new sending unit or if it had to do with that vent line which is what triggered this question.
Click to expand...

Without a vent the tank will build pressure when filling and may want to kick gas out the filler. Or, when running, the pump may try to collapse the tank in. It could hurt performance if the pump is trying to collapse the tank, but that would be in extreme cases. Vents on the 72 were part of the evaporative control system (ECS) and that’s why the vent goes to the charcoal canister in the engine compartment. The canister had a purge that connected to the carb to pull fumes into carb (closed system) to reduce emission. But vents have been used for years for the reason stated above, some more elaborate than others.

I doubt the open vent has anything to do with gas mileage. These cars have small tanks (15 gals), mine doesn't take long to hit "E" with a 408 stroker! You could cut the vent line down by the frame in the engine compartment and run a combination of rubber hose and new small ID hard line to a place up toward the top of the radiator cross brace or even inside the top of the front fender out of view. But make sure you put a small filter of some type on the end of the line to keep junk out of the vent. I imagine on hot days after a long drive you've been hearing a hissing sound coming from the open vent line. No problem, but annoying. By the way, the picture shows that your gas cap is vented, but I would still fix the vent line.
 
I just bought a chrome fuel filter and a mounting bracket and mounted it to the inner fender about carb height. No problems so far and doesn't look bad. I can post photo tonight.
 
Demon 408 said:
Without a vent the tank will build pressure when filling and may want to kick gas out the filler. Or, when running, the pump may try to collapse the tank in. It could hurt performance if the pump is trying to collapse the tank, but that would be in extreme cases. Vents on the 72 were part of the evaporative control system (ECS) and that’s why the vent goes to the charcoal canister in the engine compartment. The canister had a purge that connected to the carb to pull fumes into carb (closed system) to reduce emission. But vents have been used for years for the reason stated above, some more elaborate than others.

I doubt the open vent has anything to do with gas mileage. These cars have small tanks (15 gals), mine doesn't take long to hit "E" with a 408 stroker! You could cut the vent line down by the frame in the engine compartment and run a combination of rubber hose and new small ID hard line to a place up toward the top of the radiator cross brace or even inside the top of the front fender out of view. But make sure you put a small filter of some type on the end of the line to keep junk out of the vent. I imagine on hot days after a long drive you've been hearing a hissing sound coming from the open vent line. No problem, but annoying. By the way, the picture shows that your gas cap is vented, but I would still fix the vent line.
Click to expand...

I guess I'm not understanding the reasoning behind why I had a charcoal canister AND a vented gas cap...I mean why put an emissions device that prevents hydrocarbons from venting to the atmosphere and then also have an exit for them in the gas cap?
 
I don't the think gas caps were vented in 72. There should about 4 or 5 lines coming from the tank and up the side of the body. One of those is a vent. If you don't we just run a line from the gas inlet tube on the inside of the body down to the ground along side the 2" tube.
 
billytuffnuts said:
I guess I'm not understanding the reasoning behind why I had a charcoal canister AND a vented gas cap...I mean why put an emissions device that prevents hydrocarbons from venting to the atmosphere and then also have an exit for them in the gas cap?
Click to expand...

The cap is for pressure/vacuum relief. The ECS is for reducing emission of fuel vapors. 1972 was the first year for ECS. You don't "need" the ECS system (emissions only) for performance, but the extra vent line that was connected to the charcoal canister helps when your filling the tank with gas because it gives a pressure relief point when your pumping the filler neck full of gas and the air in the tank can't escape. I'm not a gas tank expert, this is based on my experience.
 
67gtx said:
I don't the think gas caps were vented in 72. There should about 4 or 5 lines coming from the tank and up the side of the body. One of those is a vent. If you don't we just run a line from the gas inlet tube on the inside of the body down to the ground along side the 2" tube.
Click to expand...

According to the 1972 FSM the gas caps for the Dart/Demon are vented for vacuum and pressure relief.
 
Just drill a hole inside the cap, where it seals, until you vent it to the part of the cap that is not sealed. Worked for 20 years.
 
I know this is an old post however I have a similar problem my new sending unit has two lines one for fuel coming out and the second one for venting I’m doing a 1969 Dodge dart convertible. On the feeder of the fuel tank there is a second line it looks like 5/16 tube is that the vent line? Or do I bring a second line all the way to the engine Compartment just for the vent tube
 
That's not a vent, it's a return line. Your 69 should already have the vent in the filler tube. Follow it down from the side to the bottom of the trunk, should be a 1/4" tube there
 
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