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Gas cap??


Eric 70

I have a question for y'all...

On my '70, what kind of gas cap should I be using? Vented or Non-vented? I have heard that in '71 GM went to a Vented cap because of problems that the Non-vented cap produced.
Anyway, what are the pros and cons of using each?
pressurized gas

Hi Eric,
Several years ago the 78 had a problem with gasoline fumes while driving. It turned out to be a leaking hose on top of the tank. This occured during warm weather.
However now in warm weather...if the tank is nearing 1/4 I can smell fumes AGAIN! Sometimes to the point of physical difficulty. I understand about expansion, and I wonder if a vented cap will solve this problem, too. So far, I just try to keep the tank full.
I'll look forward to seeing opinions and solutions to your post.
If you have a evaporative emissions canister (should be in the left fender well), you should have a non-vented cap. I don't know for sure about if you have disconnected the canister, but I suspect that you would need to vent your tank in some mannor.

Eric, I found this at Karl's Emissions Control Home Page

EVP - Evaporative emissions control (1969)

Unlike most of the other systems listed here, this is not an exhaust control. It is used to reduce the pollution from gasoline evaporating from the gas tank and carburetor. It works by storing the gas vapors in a charcoal canister and purging the canister during cruise conditions. The carburetor and gas tank must be sealed units. This means going from a non-vented to a vented gas cap becomes illegal.

The biggest problem with these is that on non-computer controlled cars they tend to have a jungle of vacuum lines. The only times I'd have to fail people on these was because someone would get tired of dealing with all the vacuum lines and throw the whole mess away.

Currently, an auto maker is required to make these effective up to 78 degrees F. There has been some talk of requiring these systems to be effective up to 85 degrees. I don't know where that stands.

Hope this helps ya, I couldn't find anything in the "Black Book" or the "Buyer's Guide".

As I have significantly altered my car I had to rearrange the emissions stuff.
-Moved the EVAP canister to behind the left (driverside) wheel well, attached it to the large bolt that is on the side of the frame. Blocked off the two outside hose input/outputs. The center "tank" orifice was then connected to a purchased delco emissions EVAP valve which also has a connection to to manifold vacuum. As the pressure in the tank increases while the car is not running (ie heating up) the valve opens and lets the fumes go into the canister. When the car is running the vacuum opens the valve and allows the fumes to be sucked from the canister into the tank, the canister takes in air from the bottom.

Although this system prevents the carburetor from sucking the fumes from the canister it is still environmentally friendly as the fumes are "recycled" into the tank.

If the system is not working properly in your car the fumes/gas build up in the canister and overwhelms the charcoal in it creating the gas smell in your garage. moving the canister to the rear prevents the fumes from wafting in the car.

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