Gas Tank venting problem big time! C3

[Mark #44888]

I've posted in the past about getting the right tank/cap combination for my 1970 L46, non-ec car.

This morning, at the gas station, I notice that my filler neck is now 1-2" lower than it was 2 days ago, and is also tilted towards the back. In other words, I have no venting, and the tank is caving in. The brand new $200+ tank.

#@$$$#!!!!!!

What the heck is the deal with the non-vented systems? I am at a loss. I plan on calling Quanta later and talking to them about my tank, but I need some help guys. Money aside, I don't want anymore uncontained gasoline spills.

Mark

[Patrick H.]

Re: Gas Tank venting problem big time! C3

How is the vent on the upper left hand side of the tank hooked up? Is it connected to the black plastic box on the side of the tank, and that then connected to the charcoal evap canister? Or do you not have any of that on your 70?

Patrick

[Mark #44888]

Re: Gas Tank venting problem big time! C3

Patrick -

I do not have the cannister (non-eec car). 1970 non-vented. I have a return line that runs parallel to the fuel pump with the supply line, and that's it.

I will leave my cap loose until I get this figured out

[Patrick H.]

Re: Gas Tank venting problem big time! C3

I may be incorrect, but I thought that any 1970 w/o the return venting system on the left (driver's) side of the car needed a vented cap.

Patrick

[Terry M.]

Re: Gas Tank venting problem big time! C3

Patrick -- My 1970 LT1 -- no return line at all, left or right side -- has a sealed cap. Never had any problems with the system.

[Patrick H.]

Re: Gas Tank venting problem big time! C3

Noted.

[Gary S.]

Re: Gas Tank venting problem big time! C3

Maybe I am out in left field here (won't the first time) but I thought that the fuel vapor condensor box on the top of the gas tank also performed the venting function?

Gary

[Patrick H.]

Re: Gas Tank venting problem big time! C3

My same thought, as you could tell, but apparently he does not have one. I thought all 70-74s did, but as I do not have his car here in front of me, I will have to believe him that he doesn't.

Patrick

[Dennis D.]

Re: Gas Tank venting problem big time! C3

You can find this info in the archives. I have a 70 L-46. non EEC. You should have a sealed cap. No vent line on the left. There should be a vent valve on the top of the tank to the left of the filler. (about the size of a quarter).

The valve functions to release pressure, (which is needed to keep the tank from collapsing). So you'll notice some pressure. There should be a sound similar to opening a can of tennis balls when you open the fuel cap.

The valve rarely goes bad, and if your tank is new, and correct, I would think the valve is OK. You say the tank seems to be collapsing? Would seem to me there isn't any pressure being created.

I have a explanation from Joe Lucia on this, I'll try to find and post.

[Dennis D.]

Re: here's the Joe info

Re: which gas cap??
Joe Lucia -- Thursday, 2 December 1999, at 6:23 p.m.

Dennis-----

Yes, the valves do "go bad". The valve on my original owner late 69 failed. Of course, this caused no pressure build-up or vacuum problem since ALL 69s used a VENTED fuel cap. However, when the valve malfunctioned, fuel leaked from it under "sloshing" conditions and gasoline fumes could be easily detected inside the car. I've never heard of one failing in such a way that they failed to vent or relieve vacuum, but I suppose it could happen.

Generally, with Corvette fuel systems, there is the creation of vacuum in the tank as fuel is withdrawn. The pressure/vacuum valve or the vented cap allows air to enter the tank to compensate, although the tank is not maintained at ambient atmospheric pressure---some vacuum is normal. As I understand it, NON-VENTED fuel caps also allow air to enter the tank, but not be expelled from the tank(as under heat conditions when pressure might be generated in the tank). This feature is a safe-guard against tank collapse if the pressure vacuum valve fails. VENTED type fuel caps used with tanks without pressure vacuum valves(except during August-December 1969)allow either pressure to be vented from or air to enter the tank.

As far as I know, the pressure vacuum valves are not available as a service part(and definitely not available through GM). Although the are simply soldered into the tank and, presumably, could be replaced, I suspect that the valves are are purposely made unavailable to avoid folks trying to make dangerous repairs. Radiator shops, who often service fuel tanks, might be able to obtain them, but the ones that I checked with couldn't.

[Mark #44888]

Re: Gas Tank venting problem big time! C3

Since I installed this tank fairly recently, I am familiar with its' hookups. I remember seeing the quarter-sized valve on the top of the tank.

I'm suspicious of this design: otherwise, why would they have gone to a vented cap? If the pump is sucking in more gas than is being returned via the return line, there will be a vacuum condition made. It's aggravating I'm having this kind of trouble over a stupid gas tank. It's not rocket science, nor should it be...

[Chas Henderson #28127]

Re: Gas Tank venting problem big time! C3

This is probably to simplistic, but can't you put a vented cap on?

Chasman

[Dennis D.]

Re: Gas Tank venting problem big time! C3

From what your saying, vacuum doesn't seem to be your problem. Building pressure is. I'd suspect something else. Are you stock?(carb,pump,etc)?