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Vented Gas Caps #2327994
06/28/17 11:39 AM
06/28/17 11:39 AM
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dixie2bird Offline OP
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I'm having problems with a vacuum in my gas tank that's not allowing fuel to the carb. When I open my gas cap, its sucking in air. Does anyone know where to get vented gas caps for '72 Cuda 340? I found one place online, but shipping is more than the gas cap shocked

Thanks.


1972 Cuda 340 4-speed
2015 Challenger SRT 392
2015 Jeep Wrangler Unlimited Willy's
Re: Vented Gas Caps [Re: dixie2bird] #2327999
06/28/17 11:47 AM
06/28/17 11:47 AM
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72 should be vented thru the charcoal canister

Re: Vented Gas Caps [Re: dixie2bird] #2328036
06/28/17 12:46 PM
06/28/17 12:46 PM
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70Cuda383 Offline
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The tank doesn't have any vent nipples on it anywhere? I would look for them and ensure they are free and clear to vent appropriately.


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Re: Vented Gas Caps [Re: 70Cuda383] #2328087
06/28/17 02:56 PM
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Yes the simple answer is that by 1972 there were no vented gas caps and IIRC the factories were required to make changes so that vented caps could not be retrofitted easily.
So I looked it up.
From Wikipedia.
"Since 1971, all U.S. vehicles have had fully sealed fuel systems that do not vent directly to the atmosphere".

R.

Re: Vented Gas Caps [Re: dixie2bird] #2328165
06/28/17 06:05 PM
06/28/17 06:05 PM
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BDW Offline
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I had same problem, bought a $10 locking cap, drilled small hole in plastic (on inside , not visible from outside) to allow it to vent. Something like 3/32, doesn't need much

Re: Vented Gas Caps [Re: dixie2bird] #2328456
06/29/17 04:51 AM
06/29/17 04:51 AM
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Tom_440 Offline
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I use this gas cap on my 73 Cuda and it works without a charcoal canister:

pressure and vacuum relief valve Auto Zone part CST 5771

Re: Vented Gas Caps [Re: dixie2bird] #2328590
06/29/17 02:28 PM
06/29/17 02:28 PM
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Cincinnati, Ohio
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Originally Posted By dixie2bird
I'm having problems with a vacuum in my gas tank that's not allowing fuel to the carb. When I open my gas cap, its sucking in air. Does anyone know where to get vented gas caps for '72 Cuda 340? I found one place online, but shipping is more than the gas cap shocked

Thanks.


Like most have pointed out, it is not supposed to be vented. It is supposed to be a "pressure vacuum" cap like said above. It is only designed to vent in or out in a malfunction of the vent system to keep the tank from rupturing. It does not vent enough to drive the car on.

And it is normal and preferred to have a slight vacuum or pressure when you remove the cap. It might be a vacuum or it could be pressure too. Either way it is normal.

You don't want the cap to be vented because then you may be able to smell gas in your garage when parked and the temps change and the cap will leak when the tank is full and you accelerate.

I am sure you have another fuel delivery problem and I can be sure it is a restriction on the suction side of your fuel pump.
Easy to rule out a non vented fuel cap, just remove it temporary if things are not working right or loosen it.

And a 3/32" hole is not enough to vent a fuel tank, if it appears it is then your tank is venting else where also. Is a 3/32 steam of fuel enough to keep your engine running at highway speeds?...No.

The vent should run up front to the your engine compartment and terminate on top of the engine. It can be hooked to the valve cover breather, to a charcoal canister, to the air cleaner housing or to nothing at all, just up on top of the engine.
My 74 E body has had the vent line open up on top of the engine now for years and I driven the car a lot and have driven it very hard and long out at Bonneville salt flats. The ultimate fuel delivery test.

This is my factory vent line with the rubber hose on it the keep it off the alt. It normally goes from here to the air filter and charcoal canister. I was gonna connect it to my air filter housing and never got around to it. It does not smell here as the gas fumes are heavy and stay down inside the line for the most part. Important to terminate the vent line on top of the engine. Not saying this good just that has worked for over 10K miles for me. It is how your 72 would be run.


Re: Vented Gas Caps [Re: Challenger 1] #2328828
06/29/17 09:31 PM
06/29/17 09:31 PM
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Agree replacement caps today are "pressure/vacuum relief" caps, not really vented.

The problem is carb vehicles do not like to pull fuel from a tank with vacuum... a contributing factor to vapor lock. The late 60's design where there was an open pipe to atmosphere - through many twists and turns to prevent spillage - works much better. Drilling a small hole in the cap has the same benefit... and drawbacks also.

Re: Vented Gas Caps [Re: dixie2bird] #2328832
06/29/17 09:44 PM
06/29/17 09:44 PM
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Holy fecal matter

Tanks are vented, period. All of them, even today's.

How they are vented differs substantially.

Some were just an opening to atmosphere. Some via the charcoal canister. Some used the crankcase (genius there).

You need to find out how the factory vented your tank and fix your problem.

Download a copy of the FSM, it's all there in section 25, page 14


They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.
Re: Vented Gas Caps [Re: dixie2bird] #2329015
06/30/17 03:26 AM
06/30/17 03:26 AM
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383man Offline
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In 1972 it was vented either through a charcoal canister or to the eng crankcase which is easy to do. All cars up to about 1971 vented their gas tanks right to the outside either through the gas cap or a vent line off the tank or filler neck. Some of the vent lines ended up going inside a frame rail as if it had a line it had to go up and then down in a way so gas would not flow out of it to easy. On my sons 72 Dart I cut a small slot on the gas cap rubber seal. Yes of course you will smell gas in a hot garage on a hot summer day as all cars did for years and my 63 will at times as it uses a vented cap. You want to do a simple fix just make the cap vented as its no big deal. You want to do it like a factory 72 and up car then you can get a 3 way breather cap for the valve cover and run a vent line to it to vent the tank into the crankcase but be sure you have a working PCV system if you do it that way. Or you can put a charcoal canister on it and hook it up as the factory setup was because you will have to purge the charcoal canister. You can do it what ever way you prefer as its your car and its up to you. I took the charcoal canister and all the emission stuff off my sons 72 Dart and just vented the gas cap. All new car caps will vent after they build up so much pressure or vacum as its a safety release built into them but they wont just vent all the time like the older vented gas caps. Ron

Last edited by 383man; 06/30/17 03:28 AM.
Re: Vented Gas Caps [Re: dixie2bird] #2329052
06/30/17 09:10 AM
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Gas caps haven't vented all the time since 71. They have been vented via other means, usually the charcoal canister.


They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.
Re: Vented Gas Caps [Re: dixie2bird] #2329083
06/30/17 11:29 AM
06/30/17 11:29 AM
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This is how a 72 E body fuel tank is vented. There are 4 vent lines one for each corner of the tank and they connect to a vapor housing that allows the vapor that turns to liquid to drain back into the tank. That way the tank is vented no matter how the car is parked and no matter how full the tank is. I store my cars with full fuel tanks to protect the inside of the fuel tanks.

Then one line connects to it and runs up front to the motor compartment and connects to the crankcase in 1972. The vent line should be on top of the motor where it is higher than the fuel tank.

There is no reason the alter the factory vent system as it is by far the best way to do it. There is no better way to do it. It is not complicated and is really dumb to vent a car through the gas cap after the 1971 model year car. Especially on a cuda where the filler cap is down low and on the rear of the fuel tank.


Re: Vented Gas Caps [Re: Challenger 1] #2329084
06/30/17 11:33 AM
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Re: Vented Gas Caps [Re: Challenger 1] #2329482
07/01/17 03:19 AM
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This is my factory vent line with the rubber hose on it the keep it off the alt. It normally goes from here to the air filter and charcoal canister. I was gonna connect it to my air filter housing and never got around to it. It does not smell here as the gas fumes are heavy and stay down inside the line for the most part. Important to terminate the vent line on top of the engine. Not saying this good just that has worked for over 10K miles for me. It is how your 72 would be run.




You kinda lost me here as it sounds like you are saying you have the tank vent line that comes to the front of the car and would either vent to the crankcase or canister but you have it open laying on the eng ? Or do I read that wrong as it sounds like you are letting the tank vent to the outside by letting the line lay on the eng ? But you seem to be dead set against venting to atmosphere ?

I agree the factory venting is good but depending on what car I would be building depends on how I would vent it. If I was building a muscle car for performance and it has much of the factory canister setup missing I sure as heck wont waste my time finding and buying all the factory setup when I dont have to. If the car still had all of it I would use it but otherwise I would just vent it to the atmosphere like the cars were up to 1971. Or I would vent it to the crankcase which is much easier then getting all the right factory parts to run the charcoal canister and also make sure it purges right. Course if I was restoring a car to be factory correct I would do it as the factory did. But otherwise on my hotrod muscle car I would simply vent the tank by a vent line or a vented cap like the factories did and I dont mean the newer pressure/vacum release caps they started using around 1971. And you have to remember if we vent the tank with the charcoal canister you better makes ure you have the purge part of it working.

Gas tanks have to be vented to work what ever way we choose but on my hotrod I like to keep it simple and efective which ever way I decide to do it. Many hotrodders run fuel cells which are mostly vented the old fashion way but I do prefer a stock tank since they usually hold more gas and usually have good working fuel gauges. Ron

Last edited by 383man; 07/01/17 03:24 AM.






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