C3 Gas Tank Vent?

[Elm Zobens]

I am in the process (still!) of going thru the back end of my 69' Coupe- (Rebuilt differential, spindle bearings, U-Joints, brakes etc) And I have the original gas tank out and cleaned up.

The tank itself is surprising solid for a 'rust belt' car and I have no reason to replace it. I did have a problem however, that whenever I filled the tank all the way (to the bottom of the filler neck), the car would stink of fuel until I was down a couple gallons.

I am replacing the neck gasket and screws along with a new vented cap and fuel sender gasket-
When I originally dropped the tank, I did notice the top of the tank was stained from years of fuel seepage and crude to the point that the tank sticker was barely readable. After I cleaned the tank, I noticed something on the top left side, towards the front, under the left tank strap that looks like some sort of vent or pressure valve built into the tank. I have never seen this in any of the documentation I have-
Has anyone else seen one of these? Is this what may be causing my fuel smell?
The car came with a non-vented cap when I bought it- Maybe the pressure buildup in the tank forced a small amount of fuel out of this check/pressure valve?
Of course, I would never see any fuel hit the floor under the car.
Since I am using a vented cap now, should I somehow 'seal' this valve from the outside to prevent any seepage?

Any advice or comments would be greatly appreciated.
Thanks!
Elm.

[Terry M.]

Re: C3 Gas Tank Vent?

My 1970 has a small valve on the top of the tank. I thought it was on the right (as one sits in the seat) side of the top of the tank, but it has been many years since I had the tank out so I could have a bad recollection. Regardless, the valve is non-serviceable. I believe it is supposed to be a one-way valve in it that allows air to enter the tank, but liquid is not allowed to leave the tank. If the valve is defective the only solution is to replace the tank.
My car came with a sealed cap -- at least that is what the lettering on the cap says.

[Chuck R.]

Re: C3 Gas Tank Vent?

This vent topic was raised when I was inquiring about vent lines on the 1977 replacement tank I have on my 68.

My replacement tank also has this vent close to the driver's side of the fill neck.

As Terry stated, this vent should be allowing air IN only with a non-vented cap incorporated.

I would think that if it were leaking out of this vent, it would be VERY obvious by the staining of the fuel exiting. If the staining is significantly larger around the fill neck, I would lean towards the gaskets.

I guess you could try filling the tank with hot soapy water to see if evrything is tight, but of course the down side of this is that the water is more dense than gasoline and may not produce a similar weeping type leak.

Then when your done, you just rinse out the tank and dry it out real good after right? Who knows, you may just find a split seam or the like.

I tried to refresh my memory in the archives regarding my vent line question, but the server was kicking me out so if you get the chance, you might want to take a peek as well regarding comments on this vent.

Catch Ya,

Chuck

[Elm Zobens]

Re: C3 Gas Tank Vent?

So if this is a vent to allow air IN, if I switch to a vented cap and seal off this valve, I should prevent any potential leakage from this area?
Thanks!
Elm.

[Chuck R.]

Re: In theory yes

But blanking off the vent wuold be a task in it'self. It's not one of those J-B Weld quickies, it would have to probably be pulled out, blanked off and soldered up.

Before I even considered this, I would have the tank pressure tested first to confirm just where your leak is Elm.

I had this done on a so called "tight" aluminum radiator I was going to purchase.

I caughed up the dollars to have it tested PRIOR to my purchasing it, and found that it had three leaks! The deal was that if it failed, the seller would pay for the testing. He did thankfully. I'm glad I spent the $40.00 up front.

Chuck

[Joe L.]

Re: C3 Gas Tank Vent?

Elm-----

Late 1969 Corvettes, beginning about mid-August, 1969, have an integral pressure-vacuum valve located on the top left side of the fuel tank---about 6 inches to the left of the filler opening. The use of this valve continued for 1970-74 Corvettes, too. This valves provides both pressure and vacuum relief. With it, the car does not really need the vented style cap, although all 1969s were originally built with a vented style cap.

After time, the pressure-vacuum valve will fail and fuel vapors will start to permeate the car, especially when the tank is full. The valve is not available in SERVICE and would be difficult and dangerous to replace if it was available since it is soldered into the tank. Theoretically, one should be able to seal up the pressure vacuum valve (solder, JB Weld, etc.) and one should then have, effectively, the same tank configuration as 1969s built prior to August, 1969. However, I tried that when my valve failed and, for some inexplicable reason, the odors still persisted. So, I replace the tank with a new GM tank. ALL new GM tanks for 63-69 Corvettes incorporate the pressure vacuum valve. GM also instructs that when these tanks are used on any 63-69 Corvette, the 70+ style sealed fuel cap should be used.

When I replaced the tank, I also replaced the filler neck with the 70+ style filler with internal baffle, used a filler neck-to-tank gasket I fabricated from Buna-N material (instead of the stock-style cork gasket), and used the GM #3994339 non-vented gas cap. Thereafter, ALL of my gas odor problems went away PERMANENTLY and COMPLETELY.

[Elm Zobens]

Re: C3 Gas Tank Vent?

Joe,
Thank you for the info.
Since the seepage stains and crud on the top of the tank were mainly located around this valve (I didn't even know there was a valve until I scrubbed the tank clean from all the muck), I can only assume that this valve is what is causing the smell.

I've had the same dilemma with my 70' 442. The factory cap was vented, but a non-vented cap ended up on the car sometime later and I keep having fuel spit from the vent tube whenever the tank was full. Pressure would build up inside the tank from the fuel sloshing around and would force the air and fuel from the vent tube. Once I went back to the original vented style cap, my problems went away.

I may try the JB Weld method and use the vented cap I just ordered from GM on the 69. If that doesn't work, I can always go the new tank route- But since I'm a glutton for punishment, I'll take the $2.50 route first and then the $200 route later this summer when I find out it didn't work ;-)
Thanks Again!
Elm.