C3 gas tank woes

[Mark #44888]

I've had an ongoing issue w/ my '70 L46 Corvette:

1. After I bought car, tank developed a leak. I replaced tank with a OEL tank from Quantra.

2. After about 3 weeks, I noticed the tank had collapsed in on itself. This car is supposed to use the sealed, non-vented type cap, which it had.

3. The only explanation I got, was that the relief valve that's built into the tank was defective. There has been debate on whether this valve is designed to relieve pressure, or vacuum, or both. I still don't know.

4. I've had a new tank in it for awhile, and switched to a vented cap, thinking that that might avoid the previous situation. So far, it hasn't collapsed, however:

5. I'm getting a strong gas smell in my garage after I've driven it. It was warm and sunny the other day..When I removed the cap, I got a rushing-air sound and it was definitely pressure and not vacuum.

So, what's the deal? Why did one tank have a vacuum condition, and now this one has a pressure condition? And why am I getting a reeking gas smell? It's not the carb; I've checked that.

Thanks -

confused Mark

[Joe L.]

Re: C3 gas tank woes

Mark-----

ALL 1970 Corvettes used an integral pressure vacuum valve in the tank. This valve is designed to release BOTH pressure and vacuum. However, it does not relieve to ambient conditions. In other words, it allows the build-up of some pressure or vacuum to level at which the valve relieves.

You should be able to use a "sealed" fuel cap with the proper tank and not have any problem with a collapsing tank. If you do have a collapsing tank problem, then the valve must have been defective and did not release vacuum at the proper setting. There is no way to replace the valves; you must replace the entire tank. Even though, theoretically, the valve could be un-soldered and replaced (by a professional that knows how to perform such operations on a fuel tank), I know of no source for the valve as a replacement unit.

A fuel odor in the garage is very common with these older cars. It may be emanating from the fuel tank vent or, more likely, from the carburetor.

A fuel tank may build pressure or vacuum depending upon the particular conditions that are present at any particular time. The "whooosh" you hear may not be air "blowing out" when you open the cap; it may be air going into the tank to equalize the vacuum that may exist.

[Jack H.]

Re: C3 gas tank woes

The pressure situation is pretty normal... That's why they changed the gas cap in '71 to add a safeguard statement to the emboss, "REMOVE SLOWLY".

On odor, you should see there's a 'tube' in the filler hole that extends into the tank. That tube isn't there on earlier Shark cars. The tube is supposed to maintain the proper expansion space in the tank by causing the automatic shut-off valve at the gas pump to trip when you approach the bottom of the filler tube.

BUT, if you 'play games' you can 'top off' the tank by squeezing a few extra fills in... If you do this, and then store the car without driving and depleting the 'extra' gas you loaded into the tank, you can expect expanding vapors to overwhelm the vapor recovery system (charcoal cannister in the engine compartment). That WILL produce a hefty gas odor as the charcoal can only store so much in terms of gas fumes.

The only time I'll fill my '71 to the top of the tank is when I'm on the road and I KNOW I'm going to drive a reasonable distance after filling up. See if the odor you smell might be related to a 'cockpit' issue ('topping off' the tank and storing the car with the tank slightly overfilled).

[Joe L.]

Re: C3 gas tank woes

Jack-----

1970 Corvettes, except those with NA-9 for California, did not use the "extended tube" filler neck and did not have EEC.

[Jim T.]

Re: C3 gas tank woes

Mark the gas smell in your garage could be from one of the two rubber hoses that run from the rear frame to the gas tank. Could be a loose fit or crack in the hose allowing the liquid gas to escape and causing the smell. The fuel return hose over the top of the tank is just a bit difficult to change.

[Jack H.]

Comment was aimed at NA9 with VRS... *NM*

[Mark #44888]

Re: Comment was aimed at NA9 with VRS...

I changed all of the rubber fuel lines - the two at the tank and the two at the pump. There's no leak. Could it be the gas tank, when it reaches a certain internal pressure, vents itself through the valve on the tank, or perhaps the cap?

[Joe L.]

Re: Comment was aimed at NA9 with VRS...

Mark------

Yes, it could be that.