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1969 Gas Tank Venting


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#1 Wooger

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Posted 06 July 2017 - 06:00 PM

Hey guys, I have what appears to be a late 1969 gas tank in my 2-door 510 with two ports on top.  Currently, the larger of the two (on the passenger side) is connected to the filler neck which appears to be correct.  But the smaller port in the far driver side corner is also connected directly to the filler neck.  Am i missing an evap tank?  Anything wrong with the way it is currently set-up?  I also have a non-vented gas cap.  Thanks in advance, appreciate the help.



#2 datzenmike

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Posted 06 July 2017 - 07:37 PM

If you have a non vented cap you must have a fuel tank vent line to the engine compartment in addition to the fuel line to the pump. It connects to the flow guide valve which collects fumes that are drawn into the carb to be burned and... allows air into the tank to replace the gas as it is used up. If the tank is not vented it will collapse as the tank empties.


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#3 Wooger

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Posted 06 July 2017 - 08:25 PM

Thanks for the response Mike, forgot to mention the car has a ka24de swapped in so no carbs. Only tubes coming out of the tank are the two connected red to the filler neck along with feed and return lines. Maybe I'm having a brain fart and this setup is correct?

#4 Draker

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Posted 06 July 2017 - 08:36 PM

68/69 tanks do not connect to an evap. They don't have a vent line that runs to the engine bay either. They just had a clear tube that ran under the car. That's it.

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#5 Wooger

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Posted 06 July 2017 - 08:48 PM

Gotcha, appreciate it Draker. So now I guess the question is if I should re-route the smaller tube going from the tank to the filler neck to just vent to atmosphere under the car. Getting ready to install a surge tank set-up in the trunk and want to minimize as much of the fumes as possible. Thanks again for the help.

#6 Draker

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Posted 06 July 2017 - 09:43 PM

If you run it back to the filler tube, the tank won't breath at all. If your filler cap has a good seal you might vapor lock the tank. Later models had a tiny hole in the cap to prevent this.

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#7 Draker

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Posted 06 July 2017 - 09:45 PM

I just ran a new line under the car.. only problem is.. if I or the fuel attendant tops off the tank it will syphon right out that vent.. lol. Just until the tank level drops.. so I don't top it off.

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#8 datzenmike

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Posted 07 July 2017 - 05:51 AM

Tank must be vented then. Air has to get in to replace the gas sucked out. I've collapsed a tank before.


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#9 Wooger

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Posted 07 July 2017 - 08:40 AM

Tank must be vented then. Air has to get in to replace the gas sucked out. I've collapsed a tank before.

 

I agree, it must be venting from somewhere, maybe a leaky filler neck/collar?  I should probably look into that haha.  I will probably just route the smaller tube from the top of the tank out the bottom of the trunk as mentioned by Draker and seal off the leftover inlet at the filler neck.



#10 Wooger

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Posted 07 July 2017 - 10:49 AM

Now that i think about it, if my gas tank is not vented correctly (in its current state), would it create a sort of vacuum that would lower my fuel pressure under load?  The reason I'm asking is my fuel pressure will sporadically drop significantly when i get on the throttle which leads to hesitation and bucking.  My first thought was fuel slosh and a surge tank or sump set-up was in order, but now I'm thinking the bad gas tank venting is affecting my fuel pressure.  Perhaps air isn't entering the tank at a fast enough rate to replace the fuel that is being consumed? But if that was the case i wouldn't be able to explain why the tank hasn't collapsed yet. Thoughts?  Appreciate the insight everyone.



#11 mrbigtanker

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Posted 07 July 2017 - 07:24 PM

Is it venting through the cap. Also I thought the 68/69 vented through the cap hence none of the other crap. Shit I am wondering.

#12 datzenmike

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Posted 07 July 2017 - 08:03 PM

Loosen the cap and drive it.

 

If not venting you will get a huge whooosh sound when you take the cap off to refill.


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#13 Wooger

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Posted 08 July 2017 - 07:52 PM

Is it venting through the cap. Also I thought the 68/69 vented through the cap hence none of the other crap. Shit I am wondering.


Pretty sure the cap that's on right now is not venting, big "non vented" printed on it haha. Will keep you guys posted once I get back on the road. The intake manifold is currently off on the ka while I'm changing out the heater hoses.

#14 edekalil

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Posted 09 July 2017 - 05:02 AM

Loosen the cap and drive it.

If not venting you will get a huge whooosh sound when you take the cap off to refill.

Would the whooosh be going in or out? It seems I have had pressure come out of the tank.

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#15 datzenmike

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Posted 09 July 2017 - 09:08 AM

As fuel level lowers air tries to get in if not vented. Not so noticeable if you take a week to use up a tank full. Gas will just evaporate to fill the void. On long trips where the tank is emptied in a few hours there is a huge whoosh of air going in.

 

Air puffing out can only be a non vented partly filled tank in very hot? weather. Gas evaporates and builds some pressure. Normally these gasses are fed to the engine crankcase through the vent hose and the flow guide valve for storage. Next time the engine starts they are drawn out by the PCV valve and burned. The flow guide valve also allows air back into the tank as it is emptied while running.

 

Newer cars use activated charcoal canisters for storing fumes. These are sucked out by engine vacuum the next time it's started. 


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#16 Wooger

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Posted 08 August 2017 - 07:52 AM

Just a quick update.  It seems like the gas tank was not vented properly.  I unhooked the smaller hose that was going to the filler neck and let it vent out below the car (capped the leftover hole at the filler neck) and now no more loss of fuel pressure.  I still have no idea my the tank didn't collapse in on itself.  

 

Seems like problem solved for now.  Thanks for all the help on this one.



#17 Draker

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Posted 08 August 2017 - 08:02 AM

It doesn't sound like the way you routed it is correct either.. but I guess if it's working. ok?


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#18 Wooger

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Posted 08 August 2017 - 08:36 AM

It doesn't sound like the way you routed it is correct either.. but I guess if it's working. ok?

 

I'm all for getting this right haha, especially if there are only two hoses coming out of the top of my tank.  What would be the correct way to route the hoses?  Thanks.



#19 Draker

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Posted 08 August 2017 - 08:56 AM

tan/clear tube just runs under the car

 

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#20 Wooger

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Posted 08 August 2017 - 10:05 AM

Thanks for posting that Draker, that is almost exactly how mine is currently set up.  The only difference is the vent bung on my tank is located on the far corner instead of the center.