68 Fuel tank pressure
Thread Starter
Racer
Joined: Dec 2000
Posts: 296
Likes: 190
From: North Olmsted Oh
68 Fuel tank pressure
Ok, I've spent the last 30 minutes searching the archives, and can't find anything about this problem. After driving my 68 around for a while, vapor pressure starts to builld in my gas tank. And it's not just a small hiss, the pressure is so great that it practically blows the cap out of my hands. So, there's no mistaking it's pressure and not vacuum. My car has the original (style, not piece) GF432 filter, which has the return line to the tank. Best I can figure, the return line was to prevent vapor lock in the fuel line, but it seems to cause a lot of fuel to evaporate in the tank and dramatically raise pressure in the fuel tank. The main problem is that when the tank pressure builds, the car bogs on hard acceleration, and I can actually smell that rich fuel smell in the exhaust. I can pull over, release the pressure, and it'll drive fine for a while until the pressure builds back up in the tank.
Is this just an inherent problem with the 68's or is there something that can be done? I know the gas cap is vented on my car, but the venting is meant to let air in when the fuel level drops, it doesn't let pressure out when the fuel evaporates. Has anyone blocked off that fuel return line? And would that even help this problem.
Thanks for the help.
Bill.
Is this just an inherent problem with the 68's or is there something that can be done? I know the gas cap is vented on my car, but the venting is meant to let air in when the fuel level drops, it doesn't let pressure out when the fuel evaporates. Has anyone blocked off that fuel return line? And would that even help this problem.
Thanks for the help.
Bill.
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My '70 L46 does not and it's unique in that way for 1970. All the other motors had it. It was also a non-vented cap originally, which makes no sense, but that's what it shows. I put a vented cap on it.
My tank actually collapsed inwards due to vacuum. I had to buy another new tank.
Obviously, the GM engineers goofed on this
Team Owner
Joined: Jun 2000
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From: Southbound
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Racer
Joined: Jan 2005
Posts: 389
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From: Sarasota Florida
I,m with easy mike on this one, I had the same issues with my 68. When I purchased it had the see through filter on it, and a non vented cap with return line plugged, I was getting vapor lock after hard driving, I put the correct lines on it with the correct filter, put a vented cap on it and was still having the issue. The next step was to check the vent line to make sure it was flowing freely, well the car had set for a long time, noted by the black tar in the tank!, I cleaned out the vapor return line, blew it out with air, and no issues since. hope this helps.
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Thread Starter
Racer
Joined: Dec 2000
Posts: 296
Likes: 190
From: North Olmsted Oh
Well, the cap is definitely vented, and it is vented as I described. It is a one way vent, where it allows air into the tank when the fuel level drops, but it does not allow vapors out when there is pressure in the tank. How do I know this, you ask? Well, I sacrificed my body for science. There is a little hole in the bottom of the gas cap. Put your mouth around it and try blowing through. I'll bet you can't. Try it the other way, try sucking air through it, and I got virtually no resistance. So, the cap is definitely vented and functioning as it should. I wouldn't think you'd want to vent raw fuel vapors out of the tank anyway, so I can't see how a vented cap would reduce pressure in the tank.
I will, however, check out the return line to see if it is plugged. I've had this car for 7 years and this is a problem that has recently surfaced. So, it would make sense that the return line got plugged and now I'm experiencing this problem. But if people remove that line, and stop using the GF432, then you woudn't think that could be the problem either. But anyway, I'll check it and see if it is plugged by blowing some air through it (with a compressor, I'm done tasting fuel).
Thanks for the replies....
Bill
I will, however, check out the return line to see if it is plugged. I've had this car for 7 years and this is a problem that has recently surfaced. So, it would make sense that the return line got plugged and now I'm experiencing this problem. But if people remove that line, and stop using the GF432, then you woudn't think that could be the problem either. But anyway, I'll check it and see if it is plugged by blowing some air through it (with a compressor, I'm done tasting fuel).
Thanks for the replies....
Bill
Race Director
Joined: Feb 1999
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From: In Dreams There Is Truth Ohio
Get a vented cap else you will collapse your tank
Uh hell...how did this thread up end in C5? Meant to move it to C3 Tech...C5 mods...help....please move this to C3 Tech
Last edited by GDaina; May 25, 2007 at 07:38 PM.
Race Director
Joined: Nov 1999
Posts: 10,643
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From: Foxfield CO 1970 Convertible
Not true, at least in my case. I too have a vented cap and it does the exact same thing. I'm not sure if the 68 even has an evaporator cannister.
My '70 L46 does not and it's unique in that way for 1970. All the other motors had it. It was also a non-vented cap originally, which makes no sense, but that's what it shows. I put a vented cap on it.
My tank actually collapsed inwards due to vacuum. I had to buy another new tank.
Obviously, the GM engineers goofed on this
My '70 L46 does not and it's unique in that way for 1970. All the other motors had it. It was also a non-vented cap originally, which makes no sense, but that's what it shows. I put a vented cap on it.
My tank actually collapsed inwards due to vacuum. I had to buy another new tank.
Obviously, the GM engineers goofed on this
After a few months away, perhaps some training is in order...
Burning Brakes
Joined: Aug 2005
Posts: 1,126
Likes: 0
From: Hudson Falls, N.Y. 76 Vette Modified L-48
I had the same prob after purchasing my vert....I bought a vented cap but still had the prob 
I ended up drilling a 1/4" hole right thru the middle....since that day, no hiss, no bog, just great crusin'
Many will say that's just a "bandaid" and you should find out "why" it's doing it....I feel, hey, the problem's fixed, and don't refix it if it ain't broke....
Just my 2 cents....
edit: Ooooops, I kind of over stated the hole size...try 1/64th....fat fingers, no coffee
I ended up drilling a 1/4" hole right thru the middle....since that day, no hiss, no bog, just great crusin'
Many will say that's just a "bandaid" and you should find out "why" it's doing it....I feel, hey, the problem's fixed, and don't refix it if it ain't broke....
Just my 2 cents....
edit: Ooooops, I kind of over stated the hole size...try 1/64th....fat fingers, no coffee
Last edited by kb2fzq; May 27, 2007 at 06:41 AM.
Melting Slicks
Joined: Feb 2001
Posts: 3,330
Likes: 13
From: Europe , Luxembourg
Hi
I have a vented lockable fuel cap on my 68 BB, no charcoal bu...it .
This cap has a small tube right through it and it is open both ways, hence vented. Never had any problems so far.
Günther
I have a vented lockable fuel cap on my 68 BB, no charcoal bu...it .
This cap has a small tube right through it and it is open both ways, hence vented. Never had any problems so far.
Günther
Pro
Joined: Apr 2005
Posts: 618
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I don't think GM goofed. You probably don't have the correct cap. Somewhere along the line, a new one was purchased and the owner didn't pay attention to what type. Most newer caps are not vented due to emissions regulations, so that's the type that was probably purchased.
Nope. 1970 had non-vented caps. I've checked this out w/ several sources. It wouldn't be a big deal if I had an evap. cannister, like the base model, but the L-46 did not come with one. That's why I say I think it was a goof on GM's part, as they switched to vented caps in '71 or '72.
As far as internal tank pressure, the gas tank has a small relief valve built into the top of it. You can't see it unless you drop the tank. They are not repairable as far as I know.
Race Director
Joined: May 2006
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From: Dayton, Ohio
The vented cap only allows air IN not out.A vented cap wont help with pressure buildup only vacuum buildup.
edit-Oops forgot to put in my idea on the pressure.I've seen this a couple of times.One time it was the line coming from the fuel tank was not tight and was allowing the pump to suck in air and sending most of it back to the tank via the return line and pressurizing the tank.The second time I think it was a bad diaphram doing the same thing but I didnt tear the pump apart I just replaced it because it was slightly wet around the vent hole.
edit-Oops forgot to put in my idea on the pressure.I've seen this a couple of times.One time it was the line coming from the fuel tank was not tight and was allowing the pump to suck in air and sending most of it back to the tank via the return line and pressurizing the tank.The second time I think it was a bad diaphram doing the same thing but I didnt tear the pump apart I just replaced it because it was slightly wet around the vent hole.
Last edited by ...Roger...; May 26, 2007 at 08:39 AM.
Burning Brakes
Joined: Aug 2005
Posts: 1,126
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From: Hudson Falls, N.Y. 76 Vette Modified L-48
Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
Melting Slicks
Joined: Feb 2001
Posts: 3,330
Likes: 13
From: Europe , Luxembourg
Hi
Just to repeat myself, the lockable vented cap has like a hole in it. It is a small tube soldered across the cap and I can see through it, so similar to drilling a hole through the cap. No check valve system whts so ever.
I have never heard of a explosion because of this type vented cap.
The fuel cap vents to the outside or if you want possibly around the fuel tank to the floor. Can't make it into the car.
There is more smell coming from the carb and more ignition source in that area..
Günther
Just to repeat myself, the lockable vented cap has like a hole in it. It is a small tube soldered across the cap and I can see through it, so similar to drilling a hole through the cap. No check valve system whts so ever.
I have never heard of a explosion because of this type vented cap.
The fuel cap vents to the outside or if you want possibly around the fuel tank to the floor. Can't make it into the car.
There is more smell coming from the carb and more ignition source in that area..
Günther
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Gunther your probably right that the risk of explosion is slight.But I think fixing the problem of pressure in the tank by drilling a hole in the cap is a bubba fix.I just think the reason should be found and fixed correctly.
Oh and one more thought about the hole in the cap-if you have pressure in the tank and you park on a steep incline you could have gas squirting out that hole.Just my 2 cents but I really dont like rigging things to do with fuel.
Oh and one more thought about the hole in the cap-if you have pressure in the tank and you park on a steep incline you could have gas squirting out that hole.Just my 2 cents but I really dont like rigging things to do with fuel.
Melting Slicks
Joined: Feb 2001
Posts: 3,330
Likes: 13
From: Europe , Luxembourg
Hi
I get your point, fixing a problem is always better than fiddeling with it.
The locking fuel cap has not been rigged by anybody. It is sold the way I explaint it, with a hole in it.
Also parking in a recline with a full tank will not overflow through the tank refill opening because it is recessed in such a way that the fuel will never overflow regardless what angle the car is parked. ( I think the opening is somehow lower in order to keep some kind of airpockets when filled right to the lid ).
I can well live with this minor hole in this cap and didn't ever smell any fuel vapors so far, car always parked inside my garage.
Günther
I get your point, fixing a problem is always better than fiddeling with it.
The locking fuel cap has not been rigged by anybody. It is sold the way I explaint it, with a hole in it.
Also parking in a recline with a full tank will not overflow through the tank refill opening because it is recessed in such a way that the fuel will never overflow regardless what angle the car is parked. ( I think the opening is somehow lower in order to keep some kind of airpockets when filled right to the lid ).
I can well live with this minor hole in this cap and didn't ever smell any fuel vapors so far, car always parked inside my garage.
Günther