Vented Gas Cap Fuel Loss

[Richard W.]

My SWC has the original Vented Gas cap. With the price of gasoline being what it is, does anyone have a feel for how much evaorative loss of fuel might be taking place? The car is driven a couple times a week, but I am wondering about the periods when it is sitting in the garage, which is pretty warm in the summer, and potential loss of precious fuel. Would it make sense to replace the Vented cap with a Non-Vented one when the car is not driven, and swap it back when it is. I don't smell gas in the garage or have any leaks, but I have the sense that fuel is being wasted over time. Obviously, fuel evaporates from the carburetor and is replenished upon cranking, so does it make sense that the gas tank is also losing a fair amount over time? Thanks

Rick Wallis

[Edward B.]

Re: Vented Gas Cap Fuel Loss

At the rate fuel evaporates through a vented cap it is going to take quite a while for your savings to equal the cost of a new cap. Drive the car and enjoy it.

[Duke W.]

Re: Vented Gas Cap Fuel Loss

It depends on the temperature, and the higher the temperature, the greater the rate of loss. If you let the car sit for six months at 90F it might loose up to a gallon.

If you use a sealed cap, pressure will build up and could damage the tank.

In the overall scheme of things, you are best to leave well enough alone, other than to insure the garage is properly ventilated to minimize heat buildup.

...lots of other things to worry about like changing brake fluid and antifreeze on a reasonable schedule.

Duke

[Joe L.]

Re: Vented Gas Cap Fuel Loss

My SWC has the original Vented Gas cap. With the price of gasoline being what it is, does anyone have a feel for how much evaorative loss of fuel might be taking place? The car is driven a couple times a week, but I am wondering about the periods when it is sitting in the garage, which is pretty warm in the summer, and potential loss of precious fuel. Would it make sense to replace the Vented cap with a Non-Vented one when the car is not driven, and swap it back when it is. I don't smell gas in the garage or have any leaks, but I have the sense that fuel is being wasted over time. Obviously, fuel evaporates from the carburetor and is replenished upon cranking, so does it make sense that the gas tank is also losing a fair amount over time? Thanks

Rick Wallis

Rick-----


A "vented" fuel cap does not relieve to ambient pressure. So, unless there is significant pressure build-up in the tank, no venting will occur. In a static, garaged situation, I doubt that any pressure build-up or venting will even occur.

Notwithstanding the above, the tank has to be vented. 1963-L1969 tanks had no integral vents and the fuel cap functioned as both pressure and vacuum relief. L1969-74 tanks used a non-vented cap BUT they had an integral pressure vacuum valve in the tank. You CANNOT use a non-vented tank with a non-vented cap without exposing yourself to major potential problems.

[Duke W.]

Re: Vented Gas Cap Fuel Loss

The C2 fuel tank caps have no pressure or vacuum relief valves - just a vent hole and liquid vapor separator built into the cap, so they continually release hydrocarbons to the atmosphere and don't allow any pressure or vacuum buildup in the tank.

Try blowing of drawing throught he vent hole to prove to yourself.

Duke

[Joe L.]

Re: Vented Gas Cap Fuel Loss

The C2 fuel tank caps have no pressure or vacuum relief valves - just a vent hole and liquid vapor separator built into the cap, so they continually release hydrocarbons to the atmosphere and don't allow any pressure or vacuum buildup in the tank.

Try blowing of drawing throught he vent hole to prove to yourself.

Duke

Duke-----


Only the 1963 gas cap (except Z-06) was designed as you describe. After 1963 through 1969 the caps contained a pressure vacuum valve. The later caps were also SERVICE for 1963 after the GM #3820865 was discontinued and replaced by the GM #3843576 in March, 1964.

[Richard W.]

Re: Vented Gas Cap Fuel Loss

Thanks for the input. Always a pleasure to get others' opinions and the straight story.

[Duke W.]

Re: Vented Gas Cap Fuel Loss

Duke-----


Only the 1963 gas cap (except Z-06) was designed as you describe. After 1963 through 1969 the caps contained a pressure vacuum valve. The later caps were also SERVICE for 1963 after the GM #3820865 was discontinued and replaced by the GM #3843576 in March, 1964.

In the back of my mind I thought that pressure/vacuum relief valves might have been added to the cap in '67. Thanks for setting the record straight.

For typical vintage car service, I think it's a good idea to replace the simple vented '63 cap with the second design that has the pressure/vacuum relief valves and save the original for judging.

The later cap will likely reduce fuel evaporative loss while the car is in the garage, especially in hot climates.

Duke

[Stuart F.]

Re: Vented Gas Cap Fuel Loss

I currently have a vented locking cap that I purchased from the dealer when the car was new, or shortly thereafter (I don't recall). Should I assume it functions exactly like the standard 63 cap. It's a large chrome thing, of which I believe most of the heft and weight is the locking mechanism, not a pressure/relief valve. It just has the small hole in the top and is always showing signs of fuel, i.e. rainbow look on the chrome. I have the original and it's just like new too. I don't suppose the locking cap keeps fuel in any better than the non-locking.

Stu Fox

[Kevin G.]

Re: Vented Gas Cap Fuel Loss

With said stored vehicle, would it be better to have the tank filled to the top to reduce the air surface in the tank?

[Mike M.]

Re: Vented Gas Cap Fuel Loss

Hello Richard
For the relatively insignificant savings potential after the purchase of a new cap.....
I would not risk it.
A brief memory lapse of forgetting to swich back to the vented cap before a long drive could make Quanta your new best friend and not cheaply either.
HaND

[Duke W.]

Re: Vented Gas Cap Fuel Loss

With said stored vehicle, would it be better to have the tank filled to the top to reduce the air surface in the tank?

Yes. For storage there needs to be some expansion volume for the liquid. The long filler neck on coupes is sufficient, so I recommend filling them to the bottom of the neck - maybe just below on converts. due to the shorter the neck - but the engine will use some fuel on the way back from the gas station to the garage for storage, so it's probably okay to fill a convert. to the bottom of the filler neck.

The less the free surface area, the less the evaporation rate.

For typical Corvette fuel caps that have a clearly defined vent hole on the bottom, it's easy to test if the cap is free vented or has pressure/vacuum relief valves with a suitable piece of tubing and application of oral pressure or vacuum.

The pressure/vacuum relief valves typically open at 1-2 psi differential pressure.

Duke