Is this a vented gas cap?

Old July 9th, 2019, 07:49 AM
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Is this a vented gas cap?

I'm just crossing my T's and dotting my I's. When I first bought my car (and didn't know squat) I deleted my charcoal canister and capped off the vent pipe under the hood (passenger side). Every once in a while, while in the garage, I hear a strange noise and come to find that it's either air escaping or entering the gas tank around the cap.
And one day I overfilled the tank and immediately parked in the direct sunlight which caused fuel to leak out of the cap (I think it was the cap, it was a while back). So it sounds like I have a vented cap but often, when pulling the cap off, it pops air (when it's hot out) or it's hard to get off because of suction (like this morning). So that sounds like it's not vented. Any ideas?



I blew some air through the little hole in the center with a portable, automotive compressor and the gauge on the compressor didn't change appreciably. But that doesn't explain the air rushing out of the tank when the cap is removed.
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Old July 9th, 2019, 07:52 AM
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And now, looking at these pics on my computer; is that hole threaded?
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Old July 9th, 2019, 11:16 AM
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Can you post a picture of the other side of the cap?
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Old July 9th, 2019, 01:49 PM
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Will do....
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Old July 9th, 2019, 01:56 PM
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Part store guy, maturely aged, said that a vented cap only lets air in, not out. True or false?
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Old July 9th, 2019, 02:59 PM
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Those caps are usually stamped 'vented' or 'not vented'.
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Old July 9th, 2019, 03:06 PM
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It says "open slowly" so my bet is it's NOT vented.
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Old July 9th, 2019, 03:13 PM
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Originally Posted by DJS70cutlass View Post
It says "open slowly" so my bet is it's NOT vented.
That was my thought too but a guy posted on a Chevelle forum that his vented cap said "open slowly" so I'm still in the dark.
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Old July 9th, 2019, 03:36 PM
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With the cap upside down and level, fill it with liquid and see if it makes a mess.
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Old July 9th, 2019, 04:14 PM
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I thought mine has OPEN SLOWLY on the top and either VENTED or NOT VENTED on the bottom. Can't tell on yours.
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Old July 9th, 2019, 04:17 PM
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Mac - The Fuel Filler Pressure Vacuum Cap image you have posted is exactly the same as that which is described & explained on page 8-3 of the 1971 CSM - including what you're addressing as threads.
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Old July 9th, 2019, 04:35 PM
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i think it does say something onthe oppisite side of open slowly, mac can you take that cap to a wire wheel and clean up the paint a little ?
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Old July 9th, 2019, 06:20 PM
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Okay, I think I have a non-vented cap. Or at least a cap that isn't venting. I went out to check the other wording, which only says "Caution", and when pulling the cap it was a bit stuck-on and when opened it sucked in air, and quite a bit. Even with the original charcoal canister system, it would at least need to pull in air so as not to create a vacuum, yeah? Any recommendations on a replacement cap? Or will anu old off the shelf item work okay?

The CSM states: "A positive two step locking pressure vacuum type filler cap is used on all models." And that's about it. What does "vacuum type" mean?
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Old July 9th, 2019, 06:25 PM
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You should replace with the exact same cap. The matured age dude at the store was correct. It only let's outside ambient air INTO the gas tank. The fuel pump is a mechanical pump. The mechanical pump creates a vacuum in your tank as the fuel pump pulls fuel from the tank. The air space ABOVE the fuel is less dense (less pressure) than the ambient air pressure on the outside of the fuel tank. Therefore, to equalize the pressure in the fuel tank, the one way valve in the gas cap only allows air in (to equalize the pressure). It does look like your gas cap is toast.

Last edited by Vintage Chief; July 9th, 2019 at 06:26 PM. Reason: sp
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Old July 9th, 2019, 06:36 PM
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Originally Posted by Fun71 View Post
I thought mine has OPEN SLOWLY on the top and either VENTED or NOT VENTED on the bottom. Can't tell on yours.
I just got home and looked at the gas cap on my car and yep, as already posted, it says CAUTION on the other side.
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Old July 9th, 2019, 06:41 PM
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Mine says nothing. It has the letter 'S' on the top (not that it should matter what it says anywhere on the top of the gas cap as long as it is of the same type of pressure vacuum cap). And, yes, that one-way valve inside the gas cap is designed to open under a certain amount of vacuum within the tank - but at the moment I can't recall WTH that pressure differential is to be honest.


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Old July 9th, 2019, 06:51 PM
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Maybe the "S" stands for super, like on the trans, lol. I wonder if the cap can be oiled or something. I may soak it tonight, just in the little hole, and then apply my vacuum pump tomorrow.
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Old July 9th, 2019, 06:51 PM
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I believe what becomes confusing are the terms used to describe certain parts. It truly is, IMO, a Pressure Vacuum Cap. The term 'vent' is a misnomer, IMO. There's a one-way valve inside this Pressure Vacuum Cap. I would normally consider a 'vent' to be something which allows a gas to freely 'leave' an area of it's own volition. As in a true vent. But, that's just me.

But then again, I call a motor vehicle 'propeller shaft' a drive shaft. My CSM thinks otherwise.
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Old July 9th, 2019, 06:52 PM
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Hey, I wonder if I painted it shut, all those years ago. I don't know if I can soak it in anything that won't harm the rubber seal.
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Old July 9th, 2019, 06:54 PM
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Originally Posted by Vintage Chief View Post
Mine says nothing. It has the letter 'S' on the top
That stands for STANT, the aftermarket automotive parts company, so it's a parts store replacement cap. I sold a lot of those when I worked at auto parts stores in the 80s.
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Old July 9th, 2019, 06:56 PM
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Originally Posted by Macadoo View Post
I may soak it tonight, just in the little hole, and then apply my vacuum pump tomorrow.
Then tomorrow go to your local automotive parts store and purchase a new pressure vacuum fuel cap of the same design - this one has even a larger warning.

https://www.opgi.com/cutlass/BR01997/
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Old July 9th, 2019, 06:57 PM
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Originally Posted by Fun71 View Post
That stands for STANT, the aftermarket automotive parts company, so it's a parts store replacement cap. I sold a lot of those when I worked at auto parts stores in the 80s.
Thanks Kenneth. I knew I thought that 'S' stood for a manufacturer but I couldn't recall where I'd seen it.
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Old July 9th, 2019, 07:07 PM
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You prolly saw it on a lot of radiator caps over the years.
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Old July 9th, 2019, 08:05 PM
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Well huh; mine has the S in the middle as well. Not the original cap. First thing I've found that wasn't original. That is, except for that awful triple electric fuel pump setup with the scary wiring.

Summit claims this is for a '71 Supreme;

https://www.summitracing.com/parts/s...utlass-supreme
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Old July 9th, 2019, 08:30 PM
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I always double-check part numbers. Sometimes parts supply houses display an image which 'closely resembles' the part you'll receive - LMFAO!

However, the number Summit demonstrates is 10807.

Go to STANT and input your numbers for your car and you'll arrive at the STANT part number which does match the Summit part number (in this case).

The only delta I see is the depth of the cap which goes into the neck of the fuel tank. The depth of my cap is rather deep and protrudes into the tank a decent depth. The images look shallower to me - but hell, images are difficult to decipher.

And, did they change them over the years since the time mine was replaced?

Sometimes parts houses just plain PI$$ me off. Who is to say a particular manufacturer and/or supplier doesn't subcontract out the work to someone else. Sometimes it's a hit or miss ballgame.

I'd go out and measure the depth of my cap but I'm heading to bed as there is golf to be played @ 8:00am!

Last edited by Vintage Chief; July 9th, 2019 at 08:31 PM. Reason: sp
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Old July 9th, 2019, 08:36 PM
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Rockauto has it for $2.98, of course the shipping from either house will probably be $15.

I'd hit up a local auto parts store, NAPA, Autozone, Kittrel's etc. Most have them in stock in some warehouse - they stockpile this stuff. Let them do the shipping for free and you pick it up at their store.

https://www.rockauto.com/en/catalog/...+tank+cap,5900
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Old July 9th, 2019, 08:37 PM
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Summit is usually really good about matching replacement parts and, unlike Rockauto, they actually have a phone number and a realistic return policy.
Thanks for looking up the part #, I appreciate that. Looks like Advance carries it too.

https://shop.advanceautoparts.com/p/...986%7CL3*15390

Golf on a Wednesday morning? Sounds like someone is retired I hope to join that club in a few more years.
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Old July 9th, 2019, 08:43 PM
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I measured my cap. The STANT number I provided above (10807) is exactly the depth of my cap = 1.688"
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Old July 9th, 2019, 09:08 PM
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Perfect. Thanks Norm.
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Old July 9th, 2019, 10:41 PM
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Originally Posted by Macadoo View Post
Well huh; mine has the S in the middle as well. Not the original cap.
The original cap was probably left sitting on top of a gas pump sometime in the 70s.

I think back and remember many times seeing caps on top of the pump at the station, cars driving down the road with the gas cap sitting on the roof, mangled caps on the side of the road, etc.
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Old July 10th, 2019, 05:26 AM
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Originally Posted by Macadoo View Post
Okay, I think I have a non-vented cap. Or at least a cap that isn't venting. I went out to check the other wording, which only says "Caution", and when pulling the cap it was a bit stuck-on and when opened it sucked in air, and quite a bit. Even with the original charcoal canister system, it would at least need to pull in air so as not to create a vacuum, yeah? Any recommendations on a replacement cap? Or will anu old off the shelf item work okay?

The CSM states: "A positive two step locking pressure vacuum type filler cap is used on all models." And that's about it. What does "vacuum type" mean?
I think you're right. Do you have that plastic vapor vent on the other side (towards the front of the car) of the gas tank? It has 2 hoses connected to it. That vapor vent has like a small sponge like filter inside. Also, since you disconnect the charcoal canister system, all that vapors is still stuck in the gas tank and its pressurized. This is why the cap has that "CAUTION" asking you to open slowly. If you have a vented cap, then all vapors will escape thru the gas cap and you'll be smelling gas like if you're leaking gas? Im picturing if you're drink your drink with a straw and has a tiny pin hole in it. You'll be sucking your drink with air. I think your fuel system is like a straw without that tiny pinhole. If you know what I mean? You can correct me if I'm wrong. We all can learn something new.
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Old July 10th, 2019, 06:43 AM
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You should have a non-vented cap. You can pick one up at any parts store - they're all the same stuff from Stant.

If the car has a charcoal canister then it uses a non-vented cap. Earlier cars without the canister use a vented cap.

If you're having pressure problems then the charcoal venting system is clogged somewhere. Could be the canister, any one of the many lines, or even the stand pipe behind the back seat.
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Old July 10th, 2019, 02:23 PM
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Originally Posted by Fun71 View Post
The original cap was probably left sitting on top of a gas pump sometime in the 70s.

I think back and remember many times seeing caps on top of the pump at the station, cars driving down the road with the gas cap sitting on the roof, mangled caps on the side of the road, etc.
My first full time job as a kid was pumping gas for Standard; half full serve, half self serve. We had a large box of gas caps collected from the pumps. And since it was the 70s, many were the locking kind.

Originally Posted by ELY442 View Post
I think you're right. Do you have that plastic vapor vent on the other side (towards the front of the car) of the gas tank? It has 2 hoses connected to it. That vapor vent has like a small sponge like filter inside. Also, since you disconnect the charcoal canister system, all that vapors is still stuck in the gas tank and its pressurized. This is why the cap has that "CAUTION" asking you to open slowly. If you have a vented cap, then all vapors will escape thru the gas cap and you'll be smelling gas like if you're leaking gas? Im picturing if you're drink your drink with a straw and has a tiny pin hole in it. You'll be sucking your drink with air. I think your fuel system is like a straw without that tiny pinhole. If you know what I mean? You can correct me if I'm wrong. We all can learn something new.
I've read about those plastic things. I didn't see one when I was replacing all the short hoses on the front of the tank.

Originally Posted by oddball View Post
You should have a non-vented cap. You can pick one up at any parts store - they're all the same stuff from Stant.

If the car has a charcoal canister then it uses a non-vented cap. Earlier cars without the canister use a vented cap.

If you're having pressure problems then the charcoal venting system is clogged somewhere. Could be the canister, any one of the many lines, or even the stand pipe behind the back seat.
I figured the same thing but I think this replacement is the vented type that isn't working. There's a little valve inside that hole that easily pushes in. I mentioned earlier in this thread that I did have a charcoal canister but I deleted it right after I bought the car, not really understanding how the whole system works. I'll just pick up a new vented cap.
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Old July 10th, 2019, 03:38 PM
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Originally Posted by Macadoo View Post
My first full time job as a kid was pumping gas for Standard; half full serve, half self serve.
I worked at a similar Mobil station back in my school days. Half full serve, half self serve, two mechanic bays, one oil change bay, and a tire machine. Thinking back on it, we did a lot of stuff that I wouldn't trust a school kid to do these days.
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Old July 10th, 2019, 07:00 PM
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Originally Posted by Fun71 View Post
I worked at a similar Mobil station back in my school days. Half full serve, half self serve, two mechanic bays, one oil change bay, and a tire machine. Thinking back on it, we did a lot of stuff that I wouldn't trust a school kid to do these days.
Nah, we were more new-age than that, unfortunately. We had a convenient store, very small, and a car wash with the spinning brushes. Bloomington used to host a huge Corvette show; cars from all over the world. I remember filling up Vettes from Hawaii and Germany. I also remember only changing the price signs maybe twice a year for the first few years; '78-'81. I was there until '88, when I went to sell hardware, lol.
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