Vented gas cap?

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gm1236

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How can you tell a vented cap from a non vented one?
I just put a ‘70 tank in a ‘75 body and when I open the gas cap suction is present. Is this normal or will a vented cap help this? I don’t recall the other tank having any noticeable suction.
Thanks for looking.
 
The 75 vent was recirculated back to the engine. You'll need a vented cap for the 70 tank.
 
NO (Please) do NOT use vented CAPS!!!!

The first thing is, "what is" the year of the filler neck, "for sure?" There were changes of several caps through the evap/ smog/ carbon can system and you must have the correct one for the filler neck to get it to seal

Second you must have a vent. if the 70 tank you have "had" the evap system on it, there will be extra tube fittings on the tank. if not, there will be NONE. If you DO have the extra tank fittings, the 70 evap system used a vapor separator can in the trunk.

You can "see" all this by downloading the 70 service manual. There is no 75 manual for free unless you have one already that you bought. The 73 manual will be similar and will show the changes that were underway

Frankly I would modify the filler neck to utilize the 69/ earlier vent system. This is documented, again, in the service manuals over at MyMopar
 
The 70-71-to?72 system with evap. The "overfill valve" is at the end of the long tube going up front into the engine bay. If you disconnect the carbon can system up front that tube IS the tank vent. 75 has no trunk mount vapor separator. The separator in 75 is built into the tank

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The 69/ earlier and 70 without evap controls vent

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This is a VERY simple system and works very well. It consists of a tube fitting welded up high into the filler neck. That attaches to a tube that goes "up high" against the top of the rear fender, then "U" down and follows alongside the filler neck, through the floor gasket, and simple ends----open ended---inside the rear frame rail. The cap is sealed
 
see photos of the sophisticated vent tube system for a 70 Dart. The cap is NOT vented and the vent tube goes through the rubber trunk floor /filler tube gasket and into........ wait for it........ the inside of the rear frame rail, where theoretically bad things won't get sucked into the tube.
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It’s a 75 filler neck and there weren’t any vent lines hooked to the old talk. As you stated it’s built into the 75 tank. I like the early system, looks simple enough to do.
 
that little vent tube would be a ***** to bend and get right, you might see if somebody here doesn't have a 70 tube filler neck with the vent line still on it. MAKE SURE IT ISN"T A CALIFORNIA CAR as the tube will not be there including where it is made into the filler neck at the top.
 
Thanks for the photos, if I can’t find one I’m sure I can make up something simple. It’s only moving air, no liquid or pressure to deal with.
 
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This is what I did for mine and then I ran a 5/16 in line back down through the filler neck grommet after looping it up like the metal line in the previous post. I did use some gasoline proof sealer around the fittings. You have to assemble in the car. This was a mock up.
 
What type of fitting is that?
Looks perfect for the job but uncommon. I saw this post recently somewhere else on here.

One thing I don’t quite understand, the vented cap will release pressure and let it out, the evap system also absorbs the escaping fumes. I’m looking to allow air into the tank, it has to get in so the fuel can get out. Does the evap system allow for this? If not what does? This simple tube vent system will.
Do you need to plug the tube when the car is stored inside?
 
Custom . Pretty easy. Just a barb with pipe end. The other is an pipe adapter that I cut the rest of the threads off. Like these 2 pieces
image.jpg
 
One thing I don’t quite understand, the vented cap will release pressure and let it out, the evap system also absorbs the escaping fumes. I’m looking to allow air into the tank, it has to get in so the fuel can get out. Does the evap system allow for this? If not what does? This simple tube vent system will.
Do you need to plug the tube when the car is stored inside?

The caps on all the evap control years are only for "last ditch" pressure/ vacuum release, that is, tank pressure builds up they "pop off." The actual vent was done via the 1/4" tube going up front to the evap system.

So the cap pops off pressure in either direction, and the tube up front is also bi-directional, forcing fumes into the evap system, and allowing air back into the tank
 
.View attachment 1715505305View attachment 1715505304

This is what I did for mine and then I ran a 5/16 in line back down through the filler neck grommet after looping it up like the metal line in the previous post. I did use some gasoline proof sealer around the fittings. You have to assemble in the car. This was a mock up.

I've forgotten, does the neck flange on the later cars go outside or inside the fender?
 
The flange is on the outside. That is why you need to assemble after placing the filler tube into tank.
 
The flange is on the outside. That is why you need to assemble after placing the filler tube into tank.

67 is opposite, I don't remember when they switched. What I was thinking, was, on the newer stuff, you could slice one of the bolt holes "slotted" (U) on the flange on the fender. You could braze/ solder a piece of 1/4" tube into the neck and keep it "flat" IE angled into the neck. That way you could slide the tube through the "U" bolt hole on the way in. Just an idea.
 
1970 is inside the trunk with a white foam gasket - all that is outside is the screws and the filler neck pushed through from inside
 
I'm guessin they must have changed inside/ outside about when the vent system changed, and that was "some" 70s (CA only) and I believe some 71.
 
The flange is on the outside of the body. Makes it easy to get the tube in with the tank bolted up. So on the early cars what goes in first, the fill pipe or the tank? I think there would be enough room to move the flange inside. Just have to drill some new mounting holes opposite of what it is now.
 
The flange is on the outside of the body. Makes it easy to get the tube in with the tank bolted up. So on the early cars what goes in first, the fill pipe or the tank? I think there would be enough room to move the flange inside. Just have to drill some new mounting holes opposite of what it is now.

You mount the tank, then with the trunk floor gasket pushed up the filler pipe you push it in the tank gasket (giant grommet). You push it in further than it usually goes until you clear the trunk lip then pull it out into the quarter to mount. There is enough flexibility in the tank grommet to allow this. Then you slide the trunk floor gasket down the filler pipe to seal, and then you wrestle with the vent tube to get it in place (kind of like a tetras puzzle).
 
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I made up a vent, I used and old tranny cooler line. I then ran a 3/8 line through the trunk floor. I plan on plugging the vent while the car is garaged.
 
I see two problems - (1) the nipple for the vent on the stock filler tube is right inside the neck and points up, so when you put the gas nozzle in, the end is way down inside and not near the vent tube at all. It's hard to tell how far down the tube your vent is and I sure don't know the standard length of a gas pump nozzle, but it's something you might consider. (2) I presume that you will extend the vent tube to go through the floor. If you end it there, explosive gas fumes will end up in the trunk
 
I see two problems - (1) the nipple for the vent on the stock filler tube is right inside the neck and points up, so when you put the gas nozzle in, the end is way down inside and not near the vent tube at all. It's hard to tell how far down the tube your vent is and I sure don't know the standard length of a gas pump nozzle, but it's something you might consider. (2) I presume that you will extend the vent tube to go through the floor. If you end it there, explosive gas fumes will end up in the trunk
The vent is about 6” from the end, I think the fuel nozzles are longer than that. I realized that after I put it all together. I can always plug the hole and drill another if needed. I’m going to try to put gas in it today. I did vent it through the trunk floor.
 
NO (Please) do NOT use vented CAPS!!!!

The first thing is, "what is" the year of the filler neck, "for sure?" There were changes of several caps through the evap/ smog/ carbon can system and you must have the correct one for the filler neck to get it to seal

Second you must have a vent. if the 70 tank you have "had" the evap system on it, there will be extra tube fittings on the tank. if not, there will be NONE. If you DO have the extra tank fittings, the 70 evap system used a vapor separator can in the trunk.

You can "see" all this by downloading the 70 service manual. There is no 75 manual for free unless you have one already that you bought. The 73 manual will be similar and will show the changes that were underway

Frankly I would modify the filler neck to utilize the 69/ earlier vent system. This is documented, again, in the service manuals over at MyMopar
Why not use a vented cap? What's wrong with that?
 
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