Gas cap - Vented or Not

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j_anderson

j_anderson

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I am on the market for a paintable gas cap. I see vented and non-vented are available.

I feel like an idiot for asking but how do I know which fuel cap I need?

My fuel filler neck doesn't have an external line attached to it. What other info is needed?

1972 Dart Swinger 2DHT 318
 
Does your gas tank have a vent built into the top of it? Some have one, some have a few, some have none. If none you need a vented cap. I bet you have a vent tube at the tank, they are typically above the sender.

Here is a 71 to 76 tank.

Gas Tank (16 Gallon, w/ 1 Front Vent Line) - 71-76 Dodge Plymouth A-Body Gas Tanks | AMD 890-1071

upload_2021-9-28_9-47-25.png
 
dano said:
Does your gas tank have a vent built into the top of it? Some have one, some have a few, some have none. If none you need a vented cap. I bet you have a vent tube at the tank, they are typically above the sender.
Click to expand...
I have a single vent nipple on the tank
 
It depends on how your fuel system is set up and since you've not told us, none of our answers will be correct.
 
dano said:
Is it hooked up to anything? I think it should be.
Click to expand...

Restoring the car. The car is still completely disassembled. I will be putting everything back on. I have the charcoal canister that I believe the tank vent terminates to, correct?

With that vent line hooked up, I just need a non-vented cap?
 
j_anderson said:
I dont know what information is relevant to getting the correct answer. Thanks for your input
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Is your fuel system stock? Modified? Complete? Is everything there and working? Can you answer that?
 
Then I believe the correct cap for your car is a vented one. The extra nipple on the sender is the vapor return. Do you have a factory service manual for the car? It should tell you.
 
Now for the correct answer

NO A bodies used a vented cap at least between 67 and 69 NOR did they later in the "sense of the word" "vented"

In the years starting with CA in 70 SOME had "evaporative emissions controls"

These ALSO DID NOT use a true vented cap NOR was the cap used "for a vent" in normal operation. they are sometimes mistaken for a vent BUT IN REALITY they are known as "pressure vacuum" caps and had relief valves to allow pressure to equalize, as the name implies, both pressure and vacuum BUT THE caps WERE NOT "vented" in the normal sense

Up through 69 (and beyond Federally) ---Tanks were vented using a simple tube welded into the filler neck

Whichever cars had the evap emissions--and these systems CHANGED somewhat nearly every year after--THE TANKS WERE VENTED through a long 1/4" tube going from the tank up front, later incorporated into the "carbon can" system and earlier, simply teed into the valve cover breather cap. THIS VENT TUBE IS THE TANK VENT

You can confirm all this by downloading the various service manuals over at MyMopar and reading the sections on carburetion, fuel tanks and emissions

Caps changes over the years posted by a member here NONE OF THESE ARE VENTED in the "operational" sense

abody-gas-caps-67-70-70-cal-and-71-72-74-75-76-copy-jpg.jpg


dscf2705-jpg.jpg


The vent system/ evaporative emissions system

tankvent72.jpg
 
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There seems to be some discussion about when "Ma" did away with the vapor separator in the trunk. It might be that in "late 72" they went to the newer style which had NO separator in the trunk and instead was built into the tank. This is indicated of course by the lack of the separator in the trunk

73 vent system

vent73.jpg
 
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67Dart273 said:
There seems to be some discussion about when "Ma" did away with the vapor separator in the trunk. It might be that in "late 72" they went to the newer style which had NO separator in the trunk and instead was built into the tank. This is indicated of course by the lack of the separator in the trunk

73 vent system

View attachment 1715797568
Click to expand...


maybe I'll order one of each type then figure out later which is the correct one. I really just need to get them here so they are ready when its time to paint.
 
67Dart273 said:
Now for the correct answer

NO A bodies used a vented cap at least between 67 and 69 NOR did they later in the "sense of the word" "vented"

In the years starting with CA in 70 SOME had "evaporative emissions controls"

These ALSO DID NOT use a true vented cap NOR was the cap used "for a vent" in normal operation. they are sometimes mistaken for a vent BUT IN REALITY they are known as "pressure vacuum" caps and had relief valves to allow pressure to equalize, as the name implies, both pressure and vacuum BUT THE caps WERE NOT "vented" in the normal sense

Up through 69 (and beyond Federally) ---Tanks were vented using a simple tube welded into the filler neck

Whichever cars had the evap emissions--and these systems CHANGED somewhat nearly every year after--THE TANKS WERE VENTED through a long 1/4" tube going from the tank up front, later incorporated into the "carbon can" system and earlier, simply teed into the valve cover breather cap. THIS VENT TUBE IS THE TANK VENT

You can confirm all this by downloading the various service manuals over at MyMopar and reading the sections on carburetion, fuel tanks and emissions

Caps changes over the years posted by a member here NONE OF THESE ARE VENTED in the "operational" sense

View attachment 1715797565

View attachment 1715797566

The vent system/ evaporative emissions system

View attachment 1715797567
Click to expand...


My fender tag has option:
N95: Emissions Control

I'm not seeing a manual for 1972 Dart on MyMopar unfortunately. I know I had the charcoal canister in the engine bay and my tank has only one vent line nipple on the top. So, I'm going with the non-vented cap. Thanks for the information.
 
Download the 72 Plymouth one. The only differences are the obvious differences in trim and panels, and some cars have a different instrument cluster, but wiring is similar. Ralleye cluster is differnet so that is an issue for some.
 
my CA emissions 70 Dart had the "tank in a tank system" which is now recycled into something more useful, like a tin can... All the rest of my 70 Darts had the single vent tube to the filler neck that ended... wait for it.... in the frame rail under the filler tube rubber seal in the trunk.
 
oops, almost forgot the most important point - get two caps, get them both painted and put one away. If you don't, I can guarantee that you will leave the painted cap at a gas station or on top of your car within 2 weeks for getting the car painted, and you will run around with a lame chrome aftermarket replacement cap forever. If you have two painted caps, you will never lose the first one!

This is the same theory under which I pack a raincoat when hiking because if I don't the clouds will roll in and drench me before I can get back to the car.
 
str12-340 said:
oops, almost forgot the most important point - get two caps, get them both painted and put one away. If you don't, I can guarantee that you will leave the painted cap at a gas station or on top of your car within 2 weeks for getting the car painted, and you will run around with a lame chrome aftermarket replacement cap forever. If you have two painted caps, you will never lose the first one!

This is the same theory under which I pack a raincoat when hiking because if I don't the clouds will roll in and drench me before I can get back to the car.
Click to expand...
Love this idea
 
67Dart273 said:
Now for the correct answer

NO A bodies used a vented cap at least between 67 and 69 NOR did they later in the "sense of the word" "vented"

In the years starting with CA in 70 SOME had "evaporative emissions controls"

These ALSO DID NOT use a true vented cap NOR was the cap used "for a vent" in normal operation. they are sometimes mistaken for a vent BUT IN REALITY they are known as "pressure vacuum" caps and had relief valves to allow pressure to equalize, as the name implies, both pressure and vacuum BUT THE caps WERE NOT "vented" in the normal sense

Up through 69 (and beyond Federally) ---Tanks were vented using a simple tube welded into the filler neck

Whichever cars had the evap emissions--and these systems CHANGED somewhat nearly every year after--THE TANKS WERE VENTED through a long 1/4" tube going from the tank up front, later incorporated into the "carbon can" system and earlier, simply teed into the valve cover breather cap. THIS VENT TUBE IS THE TANK VENT

You can confirm all this by downloading the various service manuals over at MyMopar and reading the sections on carburetion, fuel tanks and emissions

Caps changes over the years posted by a member here NONE OF THESE ARE VENTED in the "operational" sense

View attachment 1715797565

View attachment 1715797566

The vent system/ evaporative emissions system

View attachment 1715797567
Click to expand...
It's interesting they would use what appear to be six vented caps in that picture.

...and I typed the correct answer. .....or meant to. I forgot to use the word "non" before vented. I was tired and in a hurry. I knew the 70 and up were non vented, but I was unsure of the earlier ones.
 
This is an old thread, but it comes up in Google search results.

To offer clarity, you can group which tank and which gas cap to use on the following criteria-

1970 Federal and all earlier use a tank without the 1/4" fitting located above the sending unit. They require the usage of the correct filler pipe which has the vent fitting up high and the 1/4" metal line affixed to the filler pipe and leading downward through the filler tube seal where it ends in the frame rail...just an open line.

1970 California and all 1971 A-bodies use a tank with 4-corner venting. There are 4 fittings on the driver side of the tank that lead to a vapor separator inside the trunk. There is a single line leading from the separator up front to the engine bay, where a rubber line continues to the crank breather assembly. Only the OEM breather has the fittings. On these models the carb bowl vent connects to the fuel pump housing on Slants and to the same crank breather on small-blocks.

This is a crude, poorly functioning system that caused hard starting after heat soak. It also is the beginning of the end for the average backyard mechanic to understand what he is looking at.

1972 A-bodies...all of them...all year, used a tank with a 1/4" vent fitting located above the sending unit location. The 4 corner system with the lines and the vapor separator went away on A-bodies. The line from the tank vent led up front as in the previous 2 years. The charcoal canister is added and the gimmicky crank breather idea is eliminated. The canister stores carb vapors, tank vapors and is trigged to digest them once the engine is running via vacuum signal from the carb.My Valiant is a very early '72 built 8/71 and it has a charcoal canister.

1973 to '76 are just an continuation of the '72 system. There is a change in the filler neck for 1975 and the requirement to use unleaded which required a cap change as shown above.

Use the fuel cap as outlined above. There shouldn't be any need to buy several caps to figure out which one is the correct one to use if you look at the pictures above and understand what is being said here.

If you don't want to try to understand what is being talked about here, or you simply don't wish to conform, you can build your car any way you like. But, there won't be a cap that just lets the vapors OUT of the cap unless you throw the cap away and stick in a rag in it. And, if you try to use a tank with no vent provision, i.e. no vent on the tank and a filler neck without a vent, you won't like it. It'll nearly blow the cap off when you unscrew it on a hot day and the pressure might blow the needle off the carb seat and flood it.
 
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