Fuel tank cap

[Chevy Dave]

 I have 70 SS 454 without the evaporative emissions and no charcoal canister. My fuel pump has a return line. Should I be using a vented gas cap?

[Scott S.]

David,

I'd go with the vented cap because it allows air to replace the fuel as it's being used so as not to create vacuum in the tank which can cause flow restrictions. (under hard accelerations.)

Also allows air to escape on warm days when the fuel expands.

The reasoning behind the non vented cap was due to early emission controls, where the fuel vapours were released through the filters to trap any harmful fumes from the enviroment. 

Scott.

[Chevy Dave]

Thanks Scott!

[riverdogs00]

I have noticed on my SS with a vented cap, no return line that the vacuum in the tank seems excessive after driving. I usually go and take the cap off and the the big whoosh of air and physically watch the tank and filler return to their normal shape and position.  I have changed the cap 2x now.  If I don't do that, then I hear a high pitch noise as the the cap vents very slowly. It almost sounded like  a small electric motor.  The fuel pump is a standard carter mechanical.  I'm wondering if I need to drill a small hole in the cap. 

[Scott S.]

The one bad thing with drilling a small hole in the cap is, when the tank is full and you park on just about any incline, the fuel will leak out.

I discovered this a couple months ago when the cap gasket tore and left a puddle under the filler neck.

[riverdogs00]

15 minutes ago, Scott S. said:

The one bad thing with drilling a small hole in the cap is, when the tank is full and you park on just about any incline, the fuel will leak out.

I discovered this a couple months ago when the cap gasket tore and left a puddle under the filler neck.

well, that and or acceleration.  Not wanting that.  I keep thinking there has to be an issue with the cap, but am not sure. for now, I just loosen it and retighten if I dive more than a 5-10 miles or so.

[Grant MacPherson]

I have found the vented caps leak on hard exceleration,  is that normal?

[Scott S.]

Considering the design of the filler neck being so low in relation to the tank, I'd think it's probably a common occurance. The vented caps were designed to both relieve excess pressure and allow air in to prevent vacuum lock, but because the neck is so flat, the fuel running up the neck (slosh) overpowers the venting. 

G.M. remedied that by raising the filler to above the bumper in the second generation, and eventually moving it to the quarter panel in the 4th generation. 

[Grant MacPherson]

Thanks, just keep my foot out of the carb and problem solved.  Didn't work in 71 with a 402 and still a bit of a problem now with the 70 454. Might grow up some day but probably not any time soon. 

[Scott S.]

You could always snorkel the filler tube. 

That would help with the bending down as well....  

[S.Maug]

I've got the same issue riverdogs00  with excessive pressure  after driving with my fuel tank. haven't quite figured out. but sure, it's not normal. thought it might be a plugged fuel tank vent. And it's excessive when fueling up.  drives me crazy.  

[stangeba]

Probably not of any help to you, but here are pictures of my original 1971 Monte Carlo.

Gas Cap Pictures

 

 

[S.Maug]

wow. mines a little better shape. my 70 is an SS. problem doesn't exist on my other 70 402 car.  but now back in Illinois I'm re-looking it over. My SS 454 is here at the farm in Illinois and my other 70 402 is in Florida. nice to have my favorite car always available!!!.  My SS also has an electrical discharge when using the turn signals. always something wrong when i'm sick. but keeps a person busy!

[Scott S.]

Constant temps in Florida won't effect it as much as those cold Illinois nights.

[fountain4ever]

I am new to this site. We installed a Vapor Trapper on a customer's 1972 Monte Carlo. It was super easy, 2 clamps under the package tray and we vent the fuel tank through the new Charcoal canister and then back out under the car to atmosphere. It is a very nice part and the best part - it immediately took away the gas smell. It is a fairly new product out on the market, but it is definitely something to look into. If you type vapor trapper into the internet - it pops up, so it is easy to find. I think I saw it on Summit as well. 

The one thing I always tell my customers about their vintage cars. All new cars have a charcoal canister and that is why they do not smell like gas. Most older cars do not have a charcoal canister (at least not one that is working anymore) and they is why they DO smell like gas. The gas tank needs to breathe. Most vented gas caps (in my opinion) do not vent enough to allow proper venting. That is why after a long drive when you pull in to get fuel, you get this rush of air when you take off the gas cap. There really should not be any positive pressure or vacuum is the tank is vented properly. 

[fountain4ever]

The one thing we do is to route the vent line from the tank up as high as possible, so no liquid fuel ever gets into the canister. I like mounting this under the package try because unless you look for it, you don't see it. And these are serviceable so it's a nice place for in the future to be able to get at it easily to change out the charcoal media. Here are some others that were installed. 

[Scott S.]

Nice piece of information...

Thanks Thomas, and welcome to First Gen Montes.

[fountain4ever]

Thank you for having me. 🙂

[Dtret]

Proper venting issues come up on the site occasionally so , yes this is great information. Welcome to the group and thank you. 

[fountain4ever]

I was just checking in to see if anyone has tried the Vapor Trapper and what your results were. We have installed a bunch of these and people are super happy with them. Always use a NON-VENTED gas cap!