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Gas tank venting issues

dpstark2

dpstark2

Well-Known Member
Local time
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Location
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What's the deal with venting in these tanks? I ask because I have previously had issues where the tank held a vacuum and caused me to think I was running out of gas, but yesterday I came home from a run to the Mopar Alley Rally and shortly thereafter found a puddle of gas under the car. I popped the cap and it basically flew off the car, the tank was so pressurized. It was HOT out and I had just driven about 60 miles. I have not yet tracked down the source of the leak, but I DO NOT like having fuel leaks...

I've replaced my tank, and if anyone remembers, I modified it with an in-tank pump in preparation for fuel injection. Every single line and piece of rubber is new. The tank is new. The so-called "vent line" is clear and the hard line up to the filler tube is clear. I seem to remember somewhere that this car required a special cap or something. What puzzles me is that if I blow into the backside of the cap, it vents. And it's supposed to be a vented cap.

So... any ideas why the tank seems to hold pressure? I know a couple other people with these cars, and they don't seem to have this issue.
 
Expansion and the vent lines are doing their job. How big was the puddle? 1" or a foot? Mine is all stock and does that after a run out in this hot weather. you can smell gas for a few hours after it's parked. I have never had more than a 1" or so of gas on the ground. You should have two stock vent lines one of them runs like a horseshoe in the trunk to take up the distance. You should have a non vented cap so the vent lines can do their job too. best thing to do is let it sit and the tank and gas will cool down.
 
It was a small puddle, maybe 6" diameter. However, there is no loop of vent line or anything in the trunk. The only connections on the tank are the sending unit, the main fill tube, and another line that comes off the top and connects at the top of the fill tube. This is commonly referred to as the "vent line" but I can't see how anything vents with it since it just connects back up to the fill tube. I've seen other setups from 68-72 and there are a variety of them, usually with one line connecting to a canister or the engine to allow expansion and then burn the fumes. This has none of that.

Obviously you have to let air in as gas goes out- but is the tank intended to hold pressure like that if it heats up? I've been through this care front to rear and I believe the tank is setup correctly, but I have a hard time believing it should pull a vacuum or hold pressure.
 
Get a vented gas cap, only a few bucks, and see if that helps ya out.
 
3748023-DSCN273628Custom29_zpsbf24cbe3.jpg


tank_zps8bb2c48d.jpg


Mine has done that too . I think gas gets trapped in the u bend and any excess pressure tends to drip gas. but then you say you don't have the line in the trunk. weird.
 
This is commonly referred to as the "vent line" but I can't see how anything vents with it since it just connects back up to the fill tube.
As 5.7 hemi stated, a vented cap will solve your problem.
 
Yeah since you don't have the u in the trunk try a vented cap you need some way of relieving the pressure of heat build up in the tank. Are you using the stock gas cap or is it a stant cap? I would say lucky you have the one vent line and it's clear. It's still doing it's job. Let's see some pics too. so close to me 20 mins and you have no pics? lol
 
Since your in Tracy, drive that dude on down to my house and we'll fix her right up!!
 
Yeah, it's some Autozone cap... the one that was on the car was exactly the same, but the rubber was shot and I got tired of full tanks spilling gas out the filler. So I replaced it. The thing is it was listed as being vented, and like I said, I can put my mouth on it and push air through it. This is why I'm so confused. It seems to be vented. Second, there is no U-loop in these cars unless I'm totally mistaken. I don't have pictures of the tank setup in the car, but I'll work on it if I can't sort this out.
 
I had the same problems years ago. To fix it I don't tighten the cap all the way, just leave it a little loose and it's worked fine. Probably not the correct solution but its worked for a lot of years.
 
I thought the vented cap only let air in not out
 
A vented cap is sort of a check valve where air comes in as gas goes out but not the other way - at least that's what I have seen. It's possible there is an over pressure relief built in but can't say for sure. My 67 Ranchero has the same issue with gas expansion due to heat and I'm thinking that era of car with today's fuel is at the root of the problem. The tank does sit right up next to the bed and perhaps 66 - 67 tank vents in general were not the same as the later years and just relied on vented caps. You say there is no U tube in the trunk so that pretty much confirms something is different. Emission regs put an end to vented caps so more elaborate venting was added to the tank. Seems from 68 - 71 every year is a different venting approach. Also the gas today is not what it was when the car was new or even how it was in the 80's. I have fuel related problems on carbureted cars now that were not even a consideration in the 80's. Could our car problems these days be pinned on modernization ? (fuel chemistry targeted for EFI cars). I have thought about adding some kind of venting provision for the older cars (including a charcoal canister hidden somewhere) because I also smell gas after I park the car, though I think the gas smell is from the carbs sitting on the hot intake.
 
Try running octane booster and lead substitute and see if that helps todays fuels are designed around efi cars. Try a different cap too.
 
Pulled into a cruise-in this Saturday, parked and found a puddle of gas almost immediately, probably from the vent tubes(front of gas tank). Then when parking at home and doing a walk-around I could hear a 'gurgling' sound. Got my ear down to the license plate and could hear the gas cap sucking air. Loosened the gas cap and the tank depressurized. I think I'll try a different cap?
 
One of the reasons I'm asking this is for another car I'm working- '72 Dart- that will be a daily driver. The Charger sits for most of the week, but I beat it up on the weekends. Virtually every time I park it in the garage after driving it, I smell gas too. It's the carburetor, for sure, but that's wasted gas. I don't care too much on the Charger, though I have noticed it losing appreciable amounts of gas from the tank while it sits. My '72 has a charcoal canister, but if there's some other way to not waste the gas, I'd like to know.

As for the Charger, I have no idea how the tank can't have SOME kind of positive pressure relief. I think the cap may be part of the problem, but I would really like to know how this setup is supposed to work from the factory. I'm wondering if adding that kind of U-bend in later cars off the fill tube is a solution? If a cap can solve it, that's good enough for me though.
 
So I figured... I have not been able to figure out of the cap I have is "vented" or not. It seems to have some type of over-pressure relief, but driving it around town this weekend, I popped the cap every time I stopped and had the same blow out issue.

Rock Auto lists STANT Part # 10648 as appropriate and "vented, anti-surge." Does that seem correct to everyone? Pretty cheap to try, at least.
 
Had same problem 20 years ago when I replaced my worn gas cap with a new "vented cap", on hot days gas would pour out of my frame rail from the vent tube.
I drilled a small hole in the center of the cap and no problem since.
 
I replaced my cap with a "vented anti-surge" cap by Gates, and so far it's worked fine. It was pretty hot out this weekend and I didn't notice any significant issues. For what it's worth, according to my trusted parts store guy, he verified that 66-67 Chargers list a different cap than other similar B-bodies. Even a '67 Coronet lists a different cap according to Rock Auto...

Thanks everyone!
 
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