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Old 05-30-2007, 01:28 AM
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428 78T/A 428 78T/A is offline
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Default Do I need to vent???

Hope someone will chime in with some advice. Im runnin a 428 in a 78 TA with no charcoal canister, return line, or vapor line. Car runs good but has a hell of a vacuum when I take the gas cap off. If I let the car run when Im getting gas itll stall out before I fill the tank.

THen I heard someone on here say that a gas tank can and will crush like a pop can if the vacuum gets too severe.

So as a little experiment, I took off the plug at the gas tank and just ran a straight peice of rubber hose angled up at the gas tank. Sure enough, no more vacuum noise when I take the cap off, which I expected.

Well today after running the car a while I stopped at the post office and when I came back outside the car wasnt running, it stalled. It wouldnt restart for nothin. I walked home came back about an hour later and started right up.

It doesnt stall when the vapor line is capped. It only stalls out when theres no vapor in the tank but Im afraid Ill crush the tank if I cap it. What gives? Do you think the post office had sumptin to do with all of this

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  #2  
Old 05-30-2007, 10:45 AM
TransAm525 TransAm525 is offline
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Re-install the charcoal canister. There is absolutely no performance gain by removing it, and it's a vital component in the fuel system. The canister will trap the vapors from the fuel tank and carburetor bowl and burn them at start-up and during cruise. Not only is this free fuel, but your garage won't smell like gas everytime you walk in. The sound you hear when you remove the cap is vacuum created from the fuel pump sucking the fuel from the tank with no vent to allow air into the tank to fill the void where the fuel was. If you install a return line, then you'll pressurize the tank without a vent. If you are dead set on not running a charcoal canister, then you'll need to buy an early model vented cap that allows air in and out of the tank. Your current late-model cap is called a vented cap, but it will only vent once the tank reaches a certain pressure for safety reasons. Your current cap does not allow air to enter the tank. I too removed the canister on my 78 TA (the charcoal deteriorated), but quickly installed a new one after walking into a garage that smelled like gas all the time. Since installing the new canister and vacuum lines, I have never smelled even a hint of fuel since, plus at $3.12 a gallon right now I want every molecule of fuel going to the carb, not the atmosphere.

It sounds like your event at the post office is something different (possibly vapor lock). Essentially, the fuel boils into vapor and you have to wait until the fuel condenses back into liquid before starting the engine again. If this problem persists, run a insulated carb gasket or spacer and route the fuel lines away from the exhaust manifolds.

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Old 12-07-2022, 01:22 PM
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Quote:
Originally Posted by 428 78T/A View Post
THen I heard someone on here say that a gas tank can and will crush like a pop can if the vacuum gets too severe.

So as a little experiment, I took off the plug at the gas tank and just ran a straight peice of rubber hose angled up at the gas tank. Sure enough, no more vacuum noise when I take the cap off, which I expected.

Well today after running the car a while I stopped at the post office and when I came back outside the car wasnt running, it stalled. It wouldnt restart for nothin. I walked home came back about an hour later and started right up.

It doesnt stall when the vapor line is capped. It only stalls out when theres no vapor in the tank but Im afraid Ill crush the tank if I cap it. What gives? Do you think the post office had sumptin to do with all of this
That basically says that you have a vacuum leak from the engine/canister
that is letting too much air into the intake for a proper engine restart.
(Much too lean). With no vent you had a over rich condition.

That being said, as you are not a "Engine Calibrator" I would hook the lines
up to the EVAP system as the shop manual shows for that model year.
So everything is working.

So at one time Pontiac had fuel tanks with a multiple line "collector" that
allowed tank venting and tank air replacement with a sealed gas car. All
of this is described in a proper year Pontiac Shop manual.

I was the one who posted that given the right amount of vacuum on the fuel tank the fuel tank can be sucked down to the size of a shoe box with no venting either thru a early style tank vent or a later style Carbon Canister system type venting.

Tom V.

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Old 12-07-2022, 01:22 PM
78w72 78w72 is offline
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the early 70s vented caps only vent one way, in. they dont vent out. the rubber diaphragm lets air in when needed but prevents gas fumes or raw fuel from going out.

a charcoal canister will prevent gas smell but the small amount of vapors it catches wont do anything for saving money on fuel.

the issue with stalling at the post office is also most likely not vapor lock... especially in december in PA, unless you have fuel lines touching exhaust components or something causing excessive heat.

i have a 1970 non vented tank in my 72 firebird & eliminated the evap canister, also have a 78 & 81 i eliminated the charcoal canisters, i keep 2 of the cars parked in a 2 car garage & never notice any strong gas smell so for me the canisters are not a concern nor will they save me any money on gas.

i use a tanks inc one way vent for the 72 that is safer than just running a piece of hose for a vent, keeps the tank vented to atmosphere & prevents fuel from spilling out in an accident that would flip the car on its side or roll over.

https://www.tanksinc.com/index.cfm/p...prod/prd98.htm

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Old 12-07-2022, 02:03 PM
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Just trying to learn by following, but I'm not understanding how venting a tank will cause starting issues.

The pump pumps with or without a vented tank, doesn't it?

If the tank has no vent, the pump suction will build vacuum, which OP describes, but the pump still delivered gas to the engine. OP says it ran well without a vent.

I can imagine the pump dropping flow without the tank vented, but it obviously was still moving gas when the tank was not vented. Doesn't this suggest the tank is getting some air to replace the gas pumped? Maybe the cap has a vent that will hold some vacuum, but still let air into the tank when the vacuum builds?

Is it possible the fuel pressure at the engine increases when the tank is vented? If so, how would this cause starting issues?

I am also confused by "I'm running... with no vapor line", then later "It doesn't stall when the vapor line is capped".

Might be helpful to get more details?

For example, mechanical pump? Carb or FI? Does the pump have a vapor return that is blocked, then opened? If original tank, how is it now sealed? What cap is on the tank? Are you sure it isn't allowing air into the tank?

Sorry, but I'm not following all this but it's interesting!

  #6  
Old 12-07-2022, 02:51 PM
Joe's Garage Joe's Garage is offline
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Default Guys, this is a FIFTEEN-year-old thread....

OP hasn't been on PY since 2016........

Good info, nonetheless.

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Old 12-07-2022, 03:01 PM
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Quote:
Originally Posted by Joe's Garage View Post
OP hasn't been on PY since 2016........

Good info, nonetheless.
It was brought back from the dead by a spammer whose post has been deleted.

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Old 12-07-2022, 03:28 PM
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LOL !!

Note to self - look at the post dates

Still interesting and tucked away.

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Old 12-07-2022, 04:52 PM
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Quote:
Originally Posted by Shiny View Post
Just trying to learn by following, but I'm not understanding how venting a tank will cause starting issues.

The pump pumps with or without a vented tank, doesn't it?

If the tank has no vent, the pump suction will build vacuum, which OP describes, but the pump still delivered gas to the engine. OP says it ran well without a vent.

I can imagine the pump dropping flow without the tank vented, but it obviously was still moving gas when the tank was not vented. Doesn't this suggest the tank is getting some air to replace the gas pumped? Maybe the cap has a vent that will hold some vacuum, but still let air into the tank when the vacuum builds?

Is it possible the fuel pressure at the engine increases when the tank is vented? If so, how would this cause starting issues?

I am also confused by "I'm running... with no vapor line", then later "It doesn't stall when the vapor line is capped".

Might be helpful to get more details?

For example, mechanical pump? Carb or FI? Does the pump have a vapor return that is blocked, then opened? If original tank, how is it now sealed? What cap is on the tank? Are you sure it isn't allowing air into the tank?

Sorry, but I'm not following all this but it's interesting!
For a fuel system to flow fuel a couple of things need to happen:

1) Atmospheric Pressure and a flow path to a lower pressure area or a different altitude for the fuel source.

Example. Some of the old Ford Model Ts and Model As did not use a engine driven fuel pump. They used a Gravity Feed System. (The fuel tank was mounted higher vs the card on the engine). A friend had a fuel line fail at midnight on the afternoon shift and he had a 40 mile drive back to his home.

So we disconnected the inlet side of the fuel pump on his vehicle, plugged the fuel line from the fuel tank, ran a line from a 5 gallon fuel can (mounted & taped down on the roof of his car), and he started the car and drove home at 20 mph on back roads to his house.

He had a shop fix his leaking fuel line and he was able to borrow a vehicle for a day to make it to work.

But our "jerry rig" fuel system still required that we leave the fuel cap loose so that atmospheric pressure could allow the fuel to flow from the fuel can to the fuel pump inlet.

Been there done that. And yes the Tanks Inc tank vent is not a good one.

Trapped air in the tank still allows some fuel to be pulled from the tank.

Tom V.

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Old 12-07-2022, 05:10 PM
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Well, one time we lost the gas cap on our 66 Catalina station wagon. I guy at a local Texaco service station replaced it with a non vented cap.
The gas tank caved in, got sucked up from the fuel pump.
They had to replace our gas tank.

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Old 12-07-2022, 05:34 PM
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I had a issue a couple years ago.My garage faces west and the heat caused pressure to build in the tank,pushed gas past the needle and seat in the Holley and filled a couple cycs.Thank God it did not light as it would have shoved a couple rods threw a 366 NASCAR block.I now have a cap with a 1/16 hole drilled in it when it sits in the garage.Put a vented one on when I drive.69 bird.Tom

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Old 12-07-2022, 05:35 PM
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I had a gas tank collapse on a 57 T-bird I was restoring for someone.
It was just sitting in the shop being worked on, wasn't running.
I didn't even notice as I was busy on the front of the car for days. The owner stopped in and asked if I had jacked up the car on the gas tank.
We laughed and figured it must have simply been an atmosphere change coming from sea level up to 5000 feet at my place.
Strange things.

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Old 12-07-2022, 05:59 PM
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Drill a small hole in the gas cap.

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Old 12-07-2022, 06:13 PM
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A small hole in the gas cap will help if the car is just sitting around in the garage or storage.

I made this post some years ago about gas tank venting Post # 67 in the thread.

http://forums.maxperformanceinc.com/...=831174&page=4

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Old 12-07-2022, 06:18 PM
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Interesting stuff!

What could go wrong?

I'd still like to know if venting a tank could possibly mess up starting a "known-happy" engine with an unvented tank. Still makes no sense to me unless the engine had somehow been running great with reduced pump pressure, then objected to a few higher psi at the carb inlet. Even that doesn't make sense to me for starting, as it would take a while for vacuum to build in a tank.

As to atmospheric pressure changes acting on large volumes, my favorite example are snack chip bags. Living at 6000 feet, we are blessed by chips getting saved from being crushed because they were packaged at lower altitude. A potato chip bag on our store shelves usually looks like a balloon.

Doesn't take much pressure differential to create a LOT of force on an area as big as a gas tank.

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Old 12-07-2022, 06:23 PM
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Quote:
Originally Posted by Tom Vaught View Post
A small hole in the gas cap will help if the car is just sitting around in the garage or storage.

I made this post some years ago about gas tank venting Post # 67 in the thread.

http://forums.maxperformanceinc.com/...=831174&page=4

Tom V.
Good thread, thanks.

I just replaced the tank in my LeMans so this topic caught my attention. I did retain the return line from the pump and both vents (1970) so I should be safe from turning my tank into a kleenex box for now...

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Old 12-07-2022, 07:22 PM
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Quote:
Originally Posted by Dragncar View Post
Well, one time we lost the gas cap on our 66 Catalina station wagon. I guy at a local Texaco service station replaced it with a non vented cap.
The gas tank caved in, got sucked up from the fuel pump.
They had to replace our gas tank.
My 69 convertible will make a whoosh when I stop for fuel, obviously not a vented cap. My other birds have the vent like on the tank. Car runs fine but when I park it for more than a day or two it won’t start have to crank the hell out of it til fuel gets up in the bowl car gets 21 MPG wonder how much that’ll change when I put a vented cap on it . Lol.

I’ve driven it on 5 hr trips before so it’s gotta be getting air somewhere or it wouldnt keep flowing fuel ti keep running.

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Old 12-07-2022, 08:49 PM
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If your fuel lines are not new, I have seen where the additional vacuum from the fuel pump actually "masked" the fact that the gas line connections had some porosity, (air
getting onto the fuel system from those same lines.

I had to replace a tank to fuel pump gas line one time (when I first installed my "Pusher Holley fuel pump" to feed the carb on initial start up).
I was pressurizing the line vs sucking on the line, and noticed gas underneath the vehicle.

Tom V.

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Old 12-07-2022, 08:53 PM
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Me 68 tank has a vent tap at front-right which loops high above the rear axle. Unvented cap so no pee-trail

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Old 12-08-2022, 11:05 AM
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Quote:
Originally Posted by Tom Vaught View Post
If your fuel lines are not new, I have seen where the additional vacuum from the fuel pump actually "masked" the fact that the gas line connections had some porosity, (air
getting onto the fuel system from those same lines.

I had to replace a tank to fuel pump gas line one time (when I first installed my "Pusher Holley fuel pump" to feed the carb on initial start up).
I was pressurizing the line vs sucking on the line, and noticed gas underneath the vehicle.

Tom V.
Thanks for sharing. This is definitely in the "what could go wrong" category but consequences are unclear to me.

Definitely a ticking clock WRT the leak getting worse but not sure pulling air into the fuel would cause a big issue. My guess is solubility of air in gas is low and as far as just having air bubbles, how would that cause issues? Maybe it could mess up fuel flow a little, being compressible, but gasoline is volatile and compressible too... maybe air bubbles would aggravate vapor lock? Interesting again...

Mike

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