Gas bubbling out of fuel tank. Why 2 vents? Can I cap one?

Northcoast300h

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What happens if I cap the one vent leaking and use a vented gas cap? (Currently I have a non-vented cap) Will that work?

Bought 1970 300 Hurst without fuel tank and installed a 1972 Newport gas tank. Runs good, nothing leaks when driving. Only happens when gas tank is more than 1/2 full. After approx 1 minute of it being parked, I hear gas tank percolating like a coffee pot and fuel comes out of one of the vents for about 10 seconds. It makes approx 6" round puddle. When percolating happens, I removed non-vented cap but no difference, no vacuum sound.

Maybe I put them on wrong? Circled is the vent where the gas bubbles out. Square is the 2nd vent that goes into the trunk and comes back out under car but I couldn't see exactly where. Looks like it comes out above the exhaust but doesn't leak. 3rd line is fuel to carb. 4th is return vapor line from carb to fuel tank. (pretty sure its only vapor going thru this line from carb)

Ignore loose cheap chinese stripped clamp. There's another one on there.

Thanks for any help.
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You might try attaching a COILED tube to the leaking one, to give some length of tube over which the petrol vapor might cool and condense back to a liquid state. If the vapor condenses in the tube, it may then be sucked back into the tank as liquid later when the contents cool and settle to a less agitated state.

I drive a '66 Newport with 2 vents on the tank. Be SURE that BOTH vent lines have their openings WELL ABOVE the top level of the tank, to further minimize the likelihood of agitated gasoline dripping out in some quantity under the stationary car. Living in 110F heat and driving in some of the worst of it, I too must attend closely to this issue.

When I buy the next gas tank for Mathilda, I will look into the possibility of one with BAFFLES inside to damp the waves from acceleration which agitate the petrol to a vapor state. GAS-O-line got its name because it consists of light alkanes which just barely remain liquid at temperatures we live under. Very little stimulus suffices to boil or simply shake these alkanes, mostly hexane and heptane nowadays into a gas phase at temperatures exceeding ~36 C.

You might even consider attaching a trap jar to the end of your leaky vent line, to permit condensate fuel a better place to abide than the ground under your car. Since I too should find a permanent solution to my own trouble with this, perhaps I might research and you may benefit, God-willing.
 
72 huh ??? I’m sure someone will weigh in but could it be the line to the charcoal fuel vapor separator? I’m not big on post 71s and I’m not sure when it came out but maybe that vent is just doing it’s circa 1972 job and it has no place to go but the ground. Only car I’ve ever known with a leaking tank from new was my 70 SST javelina. That pos would leak from a vent up top if you completely filled the tank. It would boil right out of the vent and I mean bad. Not a tiny spot this was a car bomb style puddle.
 
No, you can't cap one. That will make the problem worse. Better to somehow add a few to the top of the tank and have them go "somewhere", but that's not quite do-able.

I have this same problem of "vent bubbling" of gas when the tank is over half-full, particularly in hot weather. My '70 300 has two vents. For discussion purposes, there is a short one just outside the tank and clips to the crossmember, and a long one that goes up into the trunk and then back down into the frame rail. Both are in perfect condition and fully open to airflow (meaning no mud dauber wasp plugging them up).

However, my exhaust system was custom made (second owner) and it runs very close to the gas tank on both sides. THAT is the problem. The gas gets too warm, vents off, bubbles out. Sometime a little, sometimes a lot. Sometimes never when the airflow on the highway keeps thing cool enough or the weather is unusually cool, but creeping in traffic and/or shutting the car off is a sure way to get gas on the ground. :mad:

A coiled tube (moonshine, anyone? :D) won't work. There's too much gas and vapor when things get hot. Our giant, flat tanks allow for lots of sloshing to force raw fuel into the vent piping, then the expanding vapor from the heat pushes it out. Interestingly, my long tube into the frame is the worst culprit, opposite of what you'd normally think. So....maybe once that long tube is full of gas, a siphoning effect takes place with air INTO the tank coming from the short tube. No proof, just a thought.

My solution--which helped but still won't allow a full tank of gas in the Florida summertime--was to insulate the exhaust pipes and the sides of the tank using these products:
For the pipes, variations of this for different diameters and lengths: Heatshield Products Armor Exhaust Pipe Heat Shields 172254
For the tank sides, this: Heatshield Products HP Sticky Shields 180021

Finally, there's the thought of using a vented gas cap to help the situation. I have not tired this because fuel from our flat, 24 gallon tanks will be squirting out of the cap under any acceleration. Anyone else try a vented cap?
 
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Thanks for the quick responses. You guys have some great ideas. Ugh, I thought this was going to be an easy fix. My gas tank doesn't get hot at all. The 72 gas tank is more narrow, so exhaust is not near it. I think bubbling is only happening after I park it. Maybe from gas coming back into tank from gas line or vapors being released from return line. I'm afraid if I buy vented gas cap and cap 1st vent line which is the one leaking, gas will flow thru 2nd vent line which has its dump out tube over exhaust before it comes out the vented gas cap (because vented gas caps need a certain amount of pressure to open, correct?).

I don't want any liquid fuel escaping at all, so I'm going to drill a hole in my trunk to extend up the 1st vent line and second hole far away from any heat or electrical to vent. But there is pressure behind the fuel, so this probably won't work either. I don't know. I have a marketing degree, I'm not an engineer, but after working with auto engineers for 20yrs, they're not that smart either.
 
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How do you know your tank doesn't get hot enough to vaporize gas excessively? You said it only happens when parked and above 1/2 full, and you can hear percolating.....like a coffee pot. Hmmmm....last I checked, percolating means "hot"!

As for gas coming back into the tank from the gas line or the return line, that's what those lines do, and it's a big tank compared to the few ounces in those lines. You mentioned that you might have the vents on the wrong tank nipples. The vents in the tank have different "locations" inside the tank, so swap them around and see if that helps.
I actually am a mechanical engineer, and I would advise you to NOT vent your tank into your trunk, no matter how many moonshine coils you add to it. But I know that a marketing guy would probably know that, too. :thumbsup:

A vented cap allows air IN to take the place of fuel going to the engine, and is supposed to not let any vapors (or liquid) OUT. Because our tanks are vented to the air (like most gas tanks in those days), we don't need a vented cap, and as I mentioned, I don't envision any benefit from using one for the "bubbly" problem, and possible leakage of gas when 24 gallons slams back against it behind that license plate. So, forget that I mentioned a vented cap.....I was just thinking out loud.
 
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And a lot of heat heads south towards your tank from the general engine/trans/differential, plus radiated exhaust heat. So yeah, your gas ain't room temperature.

Maybe in 1970 cars just puked gas from time to time, and it was "normal". After all, just look at that short like at the front of the tank. How can that NOT bubble gas on hot days?
 
Fuel systems were changing on a yearly basis in the late 60's and 70's to address changing vapor emissions laws. Seems like your car might have had a single vent system originally, but your 72 tank has extra vents for the later vapor separator system. There should be no vents on the 72 tank open to atmosphere- they all need a hose going to somewhere. The trick is to determine what remains of your original system, and adapt your later tank accordingly.
Samples of the different systems-
1970 CAS system
Screen Shot 2020-08-21 at 2.27.53 AM.png

1970 ECS system
Screen Shot 2020-08-21 at 2.25.18 AM.png

1972 system
Screen Shot 2020-08-21 at 2.17.02 AM.png

If your car does not have the vapor seperator (it would be located in the "hump" over the rear axle), you can cap off the extra vents in the tank.
All of this information is better explained in the FSM's you can find at mymopar.com.
 
1. Main line to fuel pump.
2. Cap vent lines.
3. Drill tiny hole in gas cap.
4. Discard 100ft of useless plumbing.
 
1. Main line to fuel pump.
2. Cap vent lines.
3. Drill tiny hole in gas cap.
4. Discard 100ft of useless plumbing.

Depends on where one buys the privilege of rolling on paved roads. SOME of these bureaucrats don't appreciate good old fashioned PRACTICAL solutions such as this. I advise leaving the plumbing in place for SHOW, regardless of actual remedy.
 
Fuel systems were changing on a yearly basis in the late 60's and 70's to address changing vapor emissions laws. Seems like your car might have had a single vent system originally, but your 72 tank has extra vents for the later vapor separator system. There should be no vents on the 72 tank open to atmosphere- they all need a hose going to somewhere. The trick is to determine what remains of your original system, and adapt your later tank accordingly.
First, this is a very good post for general thinking about various fuel vent arrangements over the 68-to-72 years.

The 70 Hursts (and I would think all 300s but maybe not California cars) had the two vents that I describe in Post 4, and no Evaporation Control System (the funny looking thing in the trunk). From the 70 FSM, it states "Certain Chrysler vehicles are equipped with....." whereas the 71 FSM has "All Chrysler vehicles are equipped with.....", so yes indeed, we can see the changing times on a yearly basis exactly as you mentioned. Interesting that some cars in 70 got closed systems/vapor recovery/carb vents back to the crankcase/etc and others didn't. Again, I believe this was due to Cali.

But back to the 72 tank (which is definitely an ECS tank) installed in a 70 (definitely without the ECS plumbing), the OP might find success by studying his three vents, determining which one would have been the "return line" that goes forward to the engine, capping just THAT one, and opening the other two. Or, maybe just using the "return line" and capping the other two. Hell....my car pukes gas so what do I know? :(
 
Fuel systems were changing on a yearly basis in the late 60's and 70's to address changing vapor emissions laws. Seems like your car might have had a single vent system originally, but your 72 tank has extra vents for the later vapor separator system. There should be no vents on the 72 tank open to atmosphere- they all need a hose going to somewhere. The trick is to determine what remains of your original system, and adapt your later tank accordingly.
Samples of the different systems-
1970 CAS system
View attachment 396878
1970 ECS system
View attachment 396877
1972 system
View attachment 396879
If your car does not have the vapor seperator (it would be located in the "hump" over the rear axle), you can cap off the extra vents in the tank.
All of this information is better explained in the FSM's you can find at mymopar.com.

My ride has the first depicted system, as it should. I'm leaning hard toward getting one of those plastic charcoal traps like what I pasted the link to. That should catch enough drip.
 
Thanks for all the replies. I know it takes time to post these so I really appreciate it. Does anyone have a pic which shows the vent line info from a 1972 tank? (similar to 71NewYorkMan's first pic). I agree with Trace that a vented gas cap probably won't work but it is a last ditch idea.

The percolating is coming from where the fuel line or return line connects to the gas tank and the fuel comes out of the vent a couple seconds before and after the percolating starts. so I guess what is happening is the heated fuel near the engine takes about a minute to push the cooler gas back into the gas tank? When the hot gas finally makes it back near the gas tank, it flows much faster and overwhelms the vent lines? Since the 1st vent line is shorter than the second one, gas only flows out of the 1st until pressure is equalized?

I like the canister idea but wouldn't the canister just fill up and overflow anyhow? I'm trying to stop liquid, not vapor.

Car really stunk when I got it from carb and tank. People were afraid to get close to the car after I drove it. I changed the old Holley to a Thermoquad rebuilt by Demonsizzler but no change in smell. (although less problems overall with the TQ) No problem at full thottle but sometimes bogs out. Yeah, I know TQ's do that but I really think its because my distributor is really worn and vacuum advance isn't working at all. The Holley bogged too, but that's a separate issue. Anyhow, after I installed the return line, the smell from carb mostly went away but I still have this pesky tank problem.
 
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Would a fuel vent valve work? Would low pressure vapor escape but keep approx 5-10 seconds of puking higher pressure liquid gas in?

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The percolating is coming from where the fuel line or return line connects to the gas tank and the fuel comes out of the vent a couple seconds before and after the percolating starts. How do you have a "return line"? A 70 Hurst shouldn't have one, unless maybe a Cali car.

Since the 1st vent line is shorter than the second one, gas only flows out of the 1st until pressure is equalized? The shorter vent line either handles the puke flow adequately such that the second longer line doesn't have to puke, or it is oriented (inside the tank) differently than the longer "up into the trunk and back" line. That longer, higher line has more head pressure (backpressure) than the shorter one.

I like the canister idea but wouldn't the canister just fill up and overflow anyhow? Yes, absolutely.

Anyhow, after I installed the return line, the smell from carb mostly went away but I still have this pesky tank problem. Oh, you INSTALLED a return line to the 1972 tank from the carb? I'm confused.


Click to expand the Reply above!
 
Would a fuel vent valve work? Would low pressure vapor escape but keep approx 5-10 seconds of puking higher pressure liquid gas in?

No. It's just a vent, like the bare end of our existing vent tubes. The only difference is that it's like a snorkel valve that keeps gas in the tank if you're upside down, which means you have bigger problems than some gas on the ground on a hot day.

You can't contain pressure in the tank. It has to vent. Or puke. The arrangement of the vent piping inside the tank SHOULD prevent the puking. But it doesn't and I can't figure out why. Need cut open an old tank, or at least have a peek inside by removing the sender bung to understand a non-ECS tank.
 
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