Fuel tank & filler cap compatability

[Jeff S.]

I'm wondering if my fuel tank was replaced by a PO back in ancient times. What appears to be a relief valve -- round, perhaps 1" dia, 3/4" deep -- can be seen at the top of the tank, ~4" toward the driver's side of the filler neck. (Observing was done by looking in that direction with a dental mirror held (tightly!) in the filler neck & projecting just below the tank's top).

My filler cap, however, is an original & is marked 'vented'. It is my understanding that a post-1970 replacement tank should be fitted with a such relief valve & thus should be used with a 'sealed' cap such as used on model years '70-'74.

Garage fumes seem more predominant of late & I'm wondering if I have two sources of venting causing this. I've also experienced a 'whiff' of pressure upon removing the filler cap after driving -- never noticed this happening before in decades of midyear ownership. Happened last week after 30 minute return drive (warm day) from event at Colo Springs (6000') to my home at 7600', tank about half full at the time.

I intend to R&R the cork seal between filler neck & tank, along with the 9 sealing machine screws & filler cap gasket. Would installing a sealed cap at this point be appropriate or do I need to further verify that the tank indeed is post-'70? Thanks in advance.

Jeff

[Joe L.]

Re: Fuel tank & filler cap compatability

I'm wondering if my fuel tank was replaced by a PO back in ancient times. What appears to be a relief valve -- round, perhaps 1" dia, 3/4" deep -- can be seen at the top of the tank, ~4" toward the driver's side of the filler neck. (Observing was done by looking in that direction with a dental mirror held (tightly!) in the filler neck & projecting just below the tank's top).

My filler cap, however, is an original & is marked 'vented'. It is my understanding that a post-1970 replacement tank should be fitted with a such relief valve & thus should be used with a 'sealed' cap such as used on model years '70-'74.

Garage fumes seem more predominant of late & I'm wondering if I have two sources of venting causing this. I've also experienced a 'whiff' of pressure upon removing the filler cap after driving -- never noticed this happening before in decades of midyear ownership. Happened last week after 30 minute return drive (warm day) from event at Colo Springs (6000') to my home at 7600', tank about half full at the time.

I intend to R&R the cork seal between filler neck & tank, along with the 9 sealing machine screws & filler cap gasket. Would installing a sealed cap at this point be appropriate or do I need to further verify that the tank indeed is post-'70? Thanks in advance.

Jeff

Jeff------


Yes, GM SERVICE fuel tanks for all 63-69 Corvettes after about 1970 included an integral pressure-vacuum valve. When this tank is used, GM instructs that a sealed fuel cap should also be used. This is exactly the same as the PRODUCTION configuration for all 1970-74 Corvettes. The fuel tank with integral pressure-vacuum valve was actually introduced late in the 1969 model year. However, all 1969 Corvettes were also equipped with a vented type gas cap.

I strongly recommend that a sealed cap be used with the tanks with the integral pressure vacuum valve. For judging, just switch the cap temporarily.

[Leif A.]

Re: Fuel tank & filler cap compatability

Joe,

Some education, please. Why do you strongly recommend using a non-vented cap with the service replacement tanks with the integral pressure-vacuum valve? I have a service replacement tank in my car with the aforementioned pressure-vacuum valve and I run the original to the car ('67 L79) vented gas cap. Car runs great with no obvious issues. Am I doing any harm using the original cap?

Leif

[Joe L.]

Re: Fuel tank & filler cap compatability

Joe,

Some education, please. Why do you strongly recommend using a non-vented cap with the service replacement tanks with the integral pressure-vacuum valve? I have a service replacement tank in my car with the aforementioned pressure-vacuum valve and I run the original to the car ('67 L79) vented gas cap. Car runs great with no obvious issues. Am I doing any harm using the original cap?

Leif

Leif-------


No harm is done. However, using the non-vented cap helps to reduce fuel vapors that may waft into the passenger compartment, especially on convertibles. I'm convinced this is why GM went to the integral pressure-vacuum valve in the first place. The integral pressure vacuum valve does not change the fact that the tank vents. However, it moves the venting to a location outside of the area of the fuel filler and outside the area sealed by the the filler seal (aka "boot").

[Jeff S.]

Re: Fuel tank & filler cap compatability

Joe - if what I observed visually indeed a pressure relief valve, I will go with a sealed cap along with the preventive maintenance on gasket & machine screws. Any idea what the specs on pressure and/or vacuum are for either the valve or OEM vented caps?
Jeff

[Joe L.]

Re: Fuel tank & filler cap compatability

Joe - if what I observed visually indeed a pressure relief valve, I will go with a sealed cap along with the preventive maintenance on gasket & machine screws. Any idea what the specs on pressure and/or vacuum are for either the valve or OEM vented caps?
Jeff

Jeff------


I've never found pressure/vacuum specs on the valve or the cap.

[Leif A.]

Re: Fuel tank & filler cap compatability

Leif-------


No harm is done. However, using the non-vented cap helps to reduce fuel vapors that may waft into the passenger compartment, especially on convertibles. I'm convinced this is why GM went to the integral pressure-vacuum valve in the first place. The integral pressure vacuum valve does not change the fact that the tank vents. However, it moves the venting to a location outside of the area of the fuel filler and outside the area sealed by the the filler seal (aka "boot").

Thank you, sir, for your answer. Mine is a coupe and I don't smell any gas fumes while driving or stopped and idling...fortunately

[Duke W.]

Re: Fuel tank & filler cap compatability

Jeff------


Yes, GM SERVICE fuel tanks for all 63-69 Corvettes after about 1970 included an integral pressure-vacuum valve. When this tank is used, .

Where on the tank is this valve located? In other words, how does one ID this second design tank? I have a post '70 service replacement tank, but I don't recall seeing anything that looked like relief valve.

Typical fuel tank pressure/vacuum relief valves open at 1-2 psi. Some owners complain of fuel odor in the garage after a drive, and a a "sealed" tank design with a relief valve should prevent this.

On early tanks without the relief valves I recommend the cap with integral relief valves. If you have a tank with a relief valve either a sealed or relief valve cap should be used.

Duke

[Jeff S.]

Re: Fuel tank & filler cap compatability

[QUOTE=Duke Williams (22045);721494]Where on the tank is this valve located? In other words, how does one ID this second design tank? I have a post '70 service replacement tank, but I don't recall seeing anything that looked like relief valve.

Maybe as described in the first paragraph of my original post? It's probably easier to see inside the tank on a convertible with shorter filler neck. Perhaps by removing the rubber boot, it could be located for ID on the exterior top surface of the tank.

[Jim T.]

Re: Fuel tank & filler cap compatability

My 1970's original tank has the sealed cap and I am pretty sure years ago I saw the pressure relief valve to the left of the gas cap on top of the tank.

[Joe L.]

Re: Fuel tank & filler cap compatability

Where on the tank is this valve located? In other words, how does one ID this second design tank? I have a post '70 service replacement tank, but I don't recall seeing anything that looked like relief valve.

Typical fuel tank pressure/vacuum relief valves open at 1-2 psi. Some owners complain of fuel odor in the garage after a drive, and a a "sealed" tank design with a relief valve should prevent this.

On early tanks without the relief valves I recommend the cap with integral relief valves. If you have a tank with a relief valve either a sealed or relief valve cap should be used.

Duke

Duke------


The valve is about 4 inches to the left of the filler opening. It is seen as about half dollar size and soldered into the tank. It is nearly flush with the top of the tank. If you reach inside the tank, to the left of the filler opening you can feel the much larger body of the valve.

[Mark H.]

[Leif A.]

Re: Fuel tank & filler cap compatability

<br>
Mark,<br>
This is a 6 1/2 year old thread but I will give you my humble opinion. Whether you plan on having your car judged or not, The Quanta tank is the one you want. It's a known excellent product and will fit exactly like the original. For $245 you really only want to do this job once.

https://www.gastanks.com/1963-67-Corvette-Gas-Tank/productinfo/GM-33A/

[Joe L.]

Re: Fuel tank &amp; filler cap compatability

I need to replace the gas tank on my ‘64. It will not be judged, so no need for the Quanta tank with the logo. In reading multiple threads like this one, it sounds like a service replacement with the relief valve would be ideal. Of course, none are listed for a C2. Am I simply looking for a tank for a ‘69-74? They fit with no issues? Thanks!

Mark-------


Any 1969-74 Corvette fuel tank will fit without problem. The problem is that most 1969-74 tanks have nipples on the ends of one or both sides for fuel return and/or the EEC emissions control system. If you want a tank without the fittings but with the integral pressure-vaccum valve, obtain tank GM33G. If you want a tank without the valve obtain a GM33A.

These tanks are manufactured by Spectra Premium Industries and are terne coated, as original. I believe that Spectra manufactures the tanks for Quanta but adds the "OLA" embossment to the Quanta tanks. Actually, this embossment cannot even be seen with the spare tire carrier installed in the car.