I have a '69 Chevelle SS 396 350 HP car. The gas tank was replaced with an aftermarket tank(Quantam or something like that). The car currently has a non vented gas cap. When I take the cap off after driving it, it really releases alot of pressure. Should this car have a vented cap?

I have a 70 LS-5--M-22 car. I lost my gas cap in 70-71 and stuck on another cap from the dealer. Tank was building up pressure,and hissing when the cap was removed.
To make this easy, If you have a stock gas cap with the ears that stand up for your thumbs to twist it open & close.
What I did in 1971 was near the center of the cap, I drilled a .031- AKA 1/32" vent hole. Drill it from the bottom, near the center. So when the drill comes through the hole will be under one of the ears and you will not see the hole. Drill your hole just beside the so-called vent valve in the bottom of the cap. While at Chevellabration last week James Hinshaw was having problems with the SO-CALLED vented caps. I showed him what I did, He said WOW-- So simple but effective as hell.
And yes it will leak gasoline only if your car is upside down.
Bob ACES--AACA

Mike, your 69 SS should have a non vented cap. Perhaps there is a problem with the two vent tubes from the gas tank or perhaps they are not even there on your aftermarket unit. If you have no vent tubes or other source of venting the tank, then you will need to address the problem through the cap I guess. Good luck.

Can I just put a vented cap on?

Can I just put a vented cap on?
I dont know but it might leak when you stomp on the gas pedal? I guess you can try one and check to see if it leaks on a full tank.

I dont know but it might leak when you stomp on the gas pedal? I guess you can try one and check to see if it leaks on a full tank.

More like "pour out"
I tried that on my 69.
Same thing when I drilled a hole, gas came out every time I took off.

My car has the original cap and its a non vented.

Mine does too, now.
When I put the new tank on I didn't have the vent thingy so I capped off both vent pipes.
I tried the vented cap/drilled hole when pressure kept building up in the tank.

I got the part and have it right now. :hurray:

Maybe a smaller hole would have worked :confused:

I believe the vented cap allows air flow in ,but not out.

I believe the vented cap allows air flow in ,but not out.
I know they let gas out, at least the one I had did.
I had to have the vent connected to let air OUT.

I don't think letting air in or letting air out is the issue as much as equalizing the pressure between the inside and outside of the tank. Looking at the 69 AIM the typical sedan/convertible/sport coupe uses a non-vented cap (#3937810) where the El Camino and wagon uses a vented cap (#3931449) due to gas cap location where it can vent without gas spilling out due to normal driving.

I have a 67 El Camino and it had an incorrect non-vented cap for a long time because the filler hose had a small hole at the top that released pressure. When I replaced the tank and hose, but reused the non-vented cap, problems like you wouldn't believe. I didn't drive it for a couple of weeks and noticed fuel under the engine compartment coming out of the carb fuel line. Pressure had built up so much (1) the cap wouldn't come off [probably saved my life in the process] and (2) so much fuel had pushed by the carb and any intake valves open it filled the oil pan with gas to the point the engine would not turn over. I discovered the problem when I pulled a spark plug to check for spark and gas poured out the plug hole. I'd guess 5-8 gallons of gas/oil drained from the oil pan before we figured it was safe to remove the gas cap itself. Needless to say the non-vented cap (for coupes/sedans/convertibles) was replaced with a correct vented cap for the El Camino.

Here's the link on this issue:

http://www.chevelles.com/forums/showthread.php?t=186510&highlight=fuel+tank+pressure

IMHO going to the trouble of drilling out a cap is unnecessary, since, as I wrote, Stant does make a standard issue cap that relieves pressure on both sides, going into and out of the tank. I still get the occasional whoosh when popping the cap at the gas station, but I have no other issues and assume that what I am hearing is simply minor residual pressure that has not yet exceeded the built-in tolerances of the relief valve in the Stant cap. It is certainly not the massive whoosh I was hearing before I switched caps.

And note, I am convinced that the prior owner's removal of the fuel vapor canister and capping of the lines is what lead to the problem in the first instance. I recently attended the 2008 Road Kings car show in Burbank CA and observed at least a dozen early 70's era cars which had the vapor canister removed. It seems to have been the first thing chucked in the trash. But the resulting mismatch in fuel tank venting would certainly seem to explain many of the complaints voiced on this board.

According to Ground Up's catalog, they list 3 caps. 1 vented for an original tank, 1 non vented for an original tank, and 1 cap(doesn't say wether it's vented or non vented) for an import tank. ?

We installed a new gas tank and new GM vented gas cap on our '69 el Camino. Even with that cap, considerable pressure builds up in the tank. Not sure what's going on there. I do have a different cap that someone has drilled a fine hole in. MIght try that. With the Chevelle building pressure, you probably need a new vent filter, the one that comes off the front of the tank and attaches to the trunk floor behind the rear axle.

Just let me say this;
1) You can't compare El Camino to coupe as far a gas splashing out the back goes.

2) 69's do not have a "fuel vapor canister" so you can't compare them with other years.

No question that the filler location is different on Elcos, but the dual-vented Stant cap requires positive pressure to vent either in or out, so I would expect to see no sloshing problem on acceleration for Chevelles either -- the "out" vent definitely requires a couple of pounds of pressure before it pops open, so I can't see sloshing or overflow being an issue. In fact, I would expect that a drilled cap would be more likely to pose a sloshing problem on a Chevelle, since there would be no positive control on either air or gas leaving the tank through the drilled holes, on acceleration or otherwise. As to the absence of a vapor canister on pre-70 cars all I can say is that the Stant dual pressure cap that I installed WAS listed as the correct application for earlier non vapor canister cars, and it seems to have fixed the positive tank pressure problem that I now attribute to the prior owner's deletion of a vapor canister in my 70 Elco. Since it's the listed cap for the earlier cars it should absolutely work on any non vapor canister car. For later cars, owners may want to look whether they still have a fully operational vapor recovery system in place and consider the earlier cap if not. Drilling an aftermarket cap to relieve tank overpressure is always an option, but a $6 off-the-shelf Stant cap may solve the problem short of going to that trouble.

Getting back to the original 69 car question :D

I was just thinking about the gas coming out of mine when I tried a vented cap.
It's possible that the cap I had was defective, I didn't try taking it back and exchanging it.

Here is what vents a '69 gas tank in a Chevelle. Anti-squeek pad information too.

guys ......... you all missed one of the problems when there is no vent !!

if you drive it down the fwy far enough the pump pulling the fuel out of the tank will create a vacuum & can collapse the tank from the normal 14.7 air pressure around it !! :yes: BTDT

guys ......... you all missed one of the problems when there is no vent !!

if you drive it down the fwy far enough the pump pulling the fuel out of the tank will create a vacuum & can collapse the tank from the normal 14.7 air pressure around it !! :yes: BTDT

WRONG

When I had the vent pipes capped off on my 69 and drove down the highway/interstate/road/fwy? and stopped for gas, the tank always had a lot of positive pressure.
It would blow the cap out of my had if I didn't hang onto it.

guys ......... you all missed one of the problems when there is no vent !!

if you drive it down the fwy far enough the pump pulling the fuel out of the tank will create a vacuum & can collapse the tank from the normal 14.7 air pressure around it !! :yes: BTDTI think before that would happen you car would starve for fuel and your engine would run out of fuel:yes:
The tank will always get a positive pressure from the expansion of the gas fumes:yes:

I think before that would happen you car would starve for fuel and your engine would run out of fuel:yes:
The tank will always get a positive pressure from the expansion of the gas fumes:yes:

I was helping my son's friend work on his 9? pickup when we found the bottom of the tank crushed up.
He had just got the truck back from being loaned out so we figured the guy had ran over something.
Got another tank at the salvage yard and got it all installed (which required removing the bed :( )

A few days later, same thing again.
Dropped the tank and blew it back out with my air hose.
Then I discovered that he had put the wrong gas cap on the truck causing the tanks to implode.

guys ........... I have live the collapsed tank syndrome from no vent ...... believe me or not

it can & does happen .......... !!

you believe what ever you want ........ I'm telling you one of the things that can & does happen ..........

Oh I know it can happen (BTDT) but I'm telling you what DID happen to a 69 SS 396 Chevelle.