[fathom70ss]

I have an aftermarket plastic locking gas cap on my 70. When the tank is over half full gas spills out of the filler neck. I know nothing about the gas tank so I cant say weather its original or not, or even vented. Which cap should I buy to prevent spillage? I was thinking vented so my tank doesn't implode.

 

[redrocket1953]

My original one on my 70 did the same thing. The gasket had dried out over the years and kind of shrunk. I made a new one out of gasket material and its fine. Mine is a vented cap. Tank had two lines out, fuel and return line. I know that there is a non vented one also but I thought that was for the ones with the charcoal cannister but I'm not positive.

 

[fathom70ss]

narrowed it down to 2 gas caps that will fit.
#1: Shallow style EC & wagon, vented
#2: Shallow style w/o EEC, vented

both look identical and are the SAME part number online. Not sure why the same part listed twice with two different titles. And Randy I wish I had more info about my gas tank but I haven't taken it off yet. Is there any way to tell by looking at the underside?

 

[Alan F]

Neither should leak gasoline out, although I have suffered the same fate with locking caps - they don't seal too well for this application.

 

[fathom70ss]

As long as its a shallow gas cap that's vented. are both compatible?

 

[Alan F]

AFAIK, there is only one filler neck.

 

[Ark68SS]

There is a closed gasoline system, that has it's own vapor recovery which uses the non-vented cap. If you have at least 2 lines to your fuel tank (and they're all connected as they should be), you have a closed system.

There is an open system which relies on venting gas fumes to the atmosphere, and uses a vented cap. This is typically 1968 and older cars and some '69s.

:noway::noway::noway:

'69 and earlier have open vents off the back of the tank and use an UNVENTED cap, since the tank is already open to the atmosphere.
'70 and up were produced with a closed venting system that used vapor lines and hoses to vent the fuel tank to a canister located at the driver's front fender. Since the system is closed to atmospheric pressure, it needs a way to bring in air to keep the tank from collapsing, thus a VENTED cap.
BillL

EDIT: '70 California cars were required to have evaporative emission controls. All '71 and later cars had to meet that requirement. Sorry for the misinformation.

 

[Dean]

I was told that unlike other years the 70 Chevelle had no tank vents and uses a vented cap.
I know that the filler necks are different than they are on 71 tanks because I just put a 71 tank on my 70 and plugged the three tank vents and made my own vented cap to fit it out of two caps, a 70 cap and a 71 cap.

 

[Coppertop]

You need to find out what tank and set-up you have.

All 1970 Chevelles got a vented tank system UNLESS they were to be sold in California. Due to California emissions requirements, California cars got the Evaporative Emissions Control systems that consisted of a charcoal canister, special tank that got 3 hose connections for vapor separation (this can be identified by crawling under the car and looking at the "front" top of the gas tank.

Vented gas caps let AIR IN to prevent imploding from the suction created from the fuel pump and let excessive fuel VAPOR OUT to prevent "exploding" from expansion.

California cars were not "allowed" to vent fuel vapor OUT into the atmosphere.

Here's the kicker--good luck finding a vented cap that works properly. The reproduction "GM" ones are garbage. They don't relieve the pressure out until the tank is about ready to burst. I've modified some general aftermarket ones (reducing spring tension in the valve system) with limited luck.

 

[vetteman_72]

I don't mean to hijack the thread, but will add some insight as well as try to determine what to do....
My '70 Malibu with a 350 2 barrel carb, had no charcoal cannister, had a sender with no extra pipe outlet coming out of it- meaning just the fuel line...
I got an almost new Canadian built tank from a guy for a great price, but unfortunately it has 3 outlet nipples on the front of it, up near the top. I wanted to close these off, and replicate the original tank, welding them shut was NOT my first choice, since the tank had fuel in it at one point, although it's clean and dry inside now.
My thinking is to take it to a local shop that does a lot of hydraulic line work, and have some -6 fittings installed and cap them- the guy I got it from, installed short fuel line and stuck bolts into the open end and clamped it all tight- not the way I want it- and don't trust rubber lines with the fuel blends....

 

[Dean]

I didn't want to buy another new tank just to get the correct 70 tank and had the new tank for too many years to return it so I put rubber caps with small clamps on the three top vents. They'll probably will rot out and start "venting" in 40 years but I won't be around then to worry about it.

......
All 1970 Chevelles got a vented tank system UNLESS they were to be sold in California. ....
.........

Maybe they sent me a 70 California tank then and not a 71 tank

 

[Big White]

You need to find out what tank and set-up you have.

All 1970 Chevelles got a vented tank system UNLESS they were to be sold in California.

Not true unless you want to split hairs and exclude El Caminos from what you consider a Chevelle. Elky tanks are unvented and require a vented cap. I can't speak for station wagons but I suspect that might be the case for wagons as well.

 

[fathom70ss]

any cons to running a vented cap on a vented tank?

 

[Munch69ProTour]

any cons to running a vented cap on a vented tank?

Yes, spilling fuel every time you nail the throttle with over a 1/2 tank of gas.

My 69 Camaro has a vented cap, I plan on putting a vent on the neck with a roll over valve and then run a non-vented cap.

Cars with vented caps and tank fills under the bumper are easy to spot, they all have dirty rear license plates!

 

[fathom70ss]

Yes, spilling fuel every time you nail the throttle with over a 1/2 tank of gas.

My 69 Camaro has a vented cap, I plan on putting a vent on the neck with a roll over valve and then run a non-vented cap.

Cars with vented caps and tank fills under the bumper are easy to spot, they all have dirty rear license plates!

Just like mine. spilling fuel and a stained/dirty plate bracket.

 

[Les Saville]

I put a new tank on my 70 but I used the original vented factory cap, it's old and ugly but I trust it and it works great! My car is a Van Nuy's build but was sent from the line to the GM Mesa proving grounds and then sold in AZ so no EEC.

 

[Dean]

I put a new tank on my 70 but I used the original vented factory cap, it's old and ugly but I trust it and it works great! My car is a Van Nuy's build but was sent from the line to the GM Mesa proving grounds and then sold in AZ so no EEC.

Did your new tank have any vents Les?

 

[Les Saville]

Did your new tank have any vents Les?

No vents on the new one Dean.

 

[427L88]

eh, even a jury-rigged tank vented system on a 67 ( bent to mimick the original "still pipe") burps fuel on any straight line g force. Only difference is its directed through the vent line and not around the cap.

btw, it took a few caps to get the right depth of seal even on a non-vented. So dont assume your non vented is functional until you really know. Oddly, one from Carquest had the right depth or thickness of gasket to be 100% sealed.

 

[Blownchevelle68]

I have a Stant locking metal cap on my '70, non vented. I don't have any leakage that I can tell, although the fitment could be a little better.

I run the vented tank with lines up to a small breather around the trunk floor.