[TAQuest]

Originally Posted by RocktimusPryme

Question. How much pressure should it take to activate the fuel cap vent? Out of curiosity after work, I took it off and blew through it. You can get air through it, but Im talking red faced, ears popping amount of blowing to get it to push air through the cap. And you have to hold that pressure for a sec before it breaks through. Makes me wonder if most of my problem is a damned fuel cap.

When I fill my tank after it gets low on a hot day I have some tank pressure. Get a woosh sound when I take the cap off. It doesn't effect my fuel pressure. Seems like it's not vented and it doesn't matter.
I can remember back in 1979 my 69 Camaro did the same thing.
Then in 1986 my 68 Firebird did it too.
I think it's normal because I know my 69 Camaro and 68 Firebird were both totally original. The Camaro only had 26000 miles on it.

[RocktimusPryme]

Originally Posted by TAQuest

When I fill my tank after it gets low on a hot day I have some tank pressure. Get a woosh sound when I take the cap off. It doesn't effect my fuel pressure. Seems like it's not vented and it doesn't matter.
I can remember back in 1979 my 69 Camaro did the same thing.
Then in 1986 my 68 Firebird did it too.
I think it's normal because I know my 69 Camaro and 68 Firebird were both totally original. The Camaro only had 26000 miles on it.

You know what's weird. Mine used to do that when it was running fine, and it didn't do it the twice I filled on the day I went to the drag strip.

Idk man, like I said Im just going to put it back together with the new parts when I get them and see what happens. Im really hoping its not something stupid like the fuel pump concentric being bad or something. Or some miniscule detail in the lines that I basically have to rip them all out to find.

[pokey1]

Originally Posted by RocktimusPryme

As far as I know stock 67 tanks only have the cap as a vent point. I would like to add one of those rollover vents mentioned. I just don't have a lot of interest in draining and dropping the tank at the moment to install one. At this point because the pump is off and I cant run the car I would extra be guessing. I bought a gauge, sent in the pump and bought a regulator. When I get it back we will see whats what.

There is another test n tune on the 26th. Im hoping to have it back together by then, but its close.

I know the stock tank on my 67 GTO had a small vent that went up from the filler tube into the trunk, don't know about the firebird. I agree a fuel gauge will tell you the most. Good luck.

[78w72]

Originally Posted by RocktimusPryme

As far as I know stock 67 tanks only have the cap as a vent point. I would like to add one of those rollover vents mentioned. I just don't have a lot of interest in draining and dropping the tank at the moment to install one. At this point because the pump is off and I cant run the car I would extra be guessing. I bought a gauge, sent in the pump and bought a regulator. When I get it back we will see whats what.

There is another test n tune on the 26th. Im hoping to have it back together by then, but its close.

pretty sure any pre 1970 firebird & some other GM cars didnt have a vent in the tank, was just the cap. the pre 1970 caps i have had only vented inwards, if there was a pressure relief, it was at a very high psi, as i got the swooshing sound on hot days when removing the cap too. even had an original cap that did that so i bought a new aftermarket one & it does it too. heres a pic of the one i got that is a repro of an original style, it says right on the cap that "fuel may be under pressure, spray may cause injury" so by design these build pressure & are mainly an inward only vent. i can easily breathe in through mine so its enough of an inward vent to relieve vacuum.

the rollover vent comes in 2 types, the one i have is a remote mount that i connected to the return line of the robbmc pickup with a ~6" hose to mount higher than the tank inlet to relieve pressure. they make a in tank mount vent too but would require dropping the tank. best bet for now might be to just drill the cap. heres a link to their vents.

http://www.tanksinc.com/index.cfm/pa...cat/cat167.htm

[Formulajones]

Originally Posted by RocktimusPryme

As far as I know stock 67 tanks only have the cap as a vent point. .

Correct. The F-bodies up through 69 only used the cap for a vent, there were no other provisions on the tank. It wasn't until 1970 on California built F-bodies that an evap system was employed and vented the tank through a charcoal canister, which became standard on all models in all 50 states in 1971. Return style fuel systems were also used on these setups.

Pokey is right about the A-bodies. Before 1970 some of them had a vent system that went up into the trunk under the trunk latch. My fathers 69 GTO has one of these vent systems. His car also came factory equipped with a return style fuel system as well which I don't see on every 69 model.

Just an FYI, when we converted it to the Tanks Inc setup and an internal electric fuel pump, the vent system Tanks Inc sent along didn't work all that well. Basically a check valve that you are supposed to mount as high as you can. So mounting it as high as physically possible which is above the rearend, the highest point of the floor, is still barely higher than the top of the gas tank. This proved to not be enough and filling the tank, along with tank pressure, would push gas right out of that vent and leak all over the place. I ended up modifying the tank and converting it back to the stock 69 vent system that goes up into the trunk. No more issues.

[242177P]

Originally Posted by 78w72

pretty sure any pre 1970 firebird & some other GM cars didnt have a vent in the tank, was just the cap.

First gen Camaros didn't have a vent, but the Firebirds did. It's on the filler neck.

[Tom Vaught]

FUEL TANK GAS CAPS ARE DESIGNED TO ALLOW ATMOSPHERIC PRESSURE TO ENTER THE TANK, not let pressure out. If you did not allow the atmospheric pressure to enter the tank you would not have any fuel flow. The Tank would be in a vacuum. NOT GOOD.

At one time I did Emissions Calibration and Testing for EPA and CARB certification.

I used to have a full sized (at one time) Gas Tank that was the size of a Kleenex Box
(because the technician did not remove the fuel cap and the POWERFUL Fuel Suction Pump (used to drain the track fuel from the tank) sucked the tank down to the Kleenex Box size. Finally threw that tank away.

A typical mechanical fuel pump can create a small depression in the tank but is really counting on the atmospheric pressure (air) flowing thru the vent line or Carbon Canister
to pressurize the fuel in the tank and send it forward to the engine.

So gas caps do not vent outward, they vent inward.

The fuel tanks with multiple vents went to a vent manifold behind the seat and then to the carbon canister. The multiple lines were there so at least one vent line was open no matter what the position of the vehicle/tank was.

I have attached a couple of the Pontiac actual Vent Tube pictures (1966 and 1968).

Here is a link to more info on the vent tubes for Pontiac.
https://www.gtoforum.com/f154/gas-tank-vent-93353/

The Tanks inc Roll-over Valves seem to have a couple of different designs.

[TAQuest]

Originally Posted by 242177P

First gen Camaros didn't have a vent, but the Firebirds did. It's on the filler neck.

Now you are in my wheelhouse. That was only on 400 cars. It's one of the ways to doc a true one. Just like the pitman arm on a true 69 Z. Priceless info for a collector. Not many know and you catch it with clones.

[78w72]

Originally Posted by Tom Vaught

FUEL TANK GAS CAPS ARE DESIGNED TO ALLOW ATMOSPHERIC PRESSURE TO ENTER THE TANK, not let pressure out.

thats what i have found with the 1970 vented caps & i would guess they are the same for earlier years. vent inwards only.

so drilling a hole would only be a benefit if your cap doesnt vent inwards, according to tomV there is no need for a outwards pressure vent... there will be some slight pressure on hot days or as fuel heats up, but its not enough to be dangerous & explode the tank, im sure the engineers factored the pressure into the design of the tank.

tom v- so are you saying there needs to be some pressure in the tank to help fuel supply? i have a rollover vent on mine since i wanted to aleviate the slight pressure build up i got on hot days. im fine removing the cap slowly to release any pressure if the positive pressure will help fuel delivery, or at least not hurt it.

edit: i just tried to blow through the cap untill i was red faced & almost saw stars! & it does not vent out at all. even hit it with a air hose at about 50psi & no outward venting.

[tom s]

I had a experience a couple years ago with my 69 bird.Vented cap,no return line and non vented tank.Carb was a 3310 holley.My garage faces west and gets very hot in the afternoon sun.Went out to drive the bird,tried to start and the engine was locked up.Could not figure out what was going on.Finally pulled the plugs,cranked it over and it shot raw gas all over the engine compartment.The fuel vapors had expanded and pushed fuel past the needle and seats and filled a couple cyc with fuel.It was a factory 366 NASCAR V engine.Thank God it did not light and stuff a rod through the rare block.I drive the car with the vented cap but have another cap with a small hole drilled in it when the car is in the garage.FWIW,Tom

[RocktimusPryme]

Originally Posted by 78w72

the rollover vent comes in 2 types, the one i have is a remote mount that i connected to the return line of the robbmc pickup with a ~6" hose to mount higher than the tank inlet to relieve pressure. they make a in tank mount vent too but would require dropping the tank. best bet for now might be to just drill the cap. heres a link to their vents.

http://www.tanksinc.com/index.cfm/pa...cat/cat167.htm

But in order to do that you need a blank return line right? I was planning on using that port in the pickup when I get the 1100 pump back. Could you put a T in near the tank and mount a remote one like this higher up.

http://www.tanksinc.com/index.cfm/pa...prod/prd98.htm

I was actually looking to see if I could drill out the center of a cap and add one of those bulkhead style vents in the cap, but I don't think there is enough room.

[RocktimusPryme]

I have a repro tank which is probably based on a Camaro. I may get aggressive and drop it since I don't have a functioning fuel system to drive it anyway. I just don't want to deal with draining it. But I may inspect to see if there is easy room to add a bulkhead fitting and a remove vent.

My cap I did blow out not knowing any better and air did escape when you applied enough pressure, so maybe that's the problem. Again, its hard to say when I cant troubleshoot since the pump isn't in it. Mine for sure used to hold some pressure on hot days and recently it hasn't. I'll see if air moves from the other way when I get home. That said maybe it was clogged and a removed the obstruction when I blew in it last night. Its all conjecture until I get the pump back and can see whats up.

[RocktimusPryme]

I have a repro tank which is probably based on a Camaro. I may get aggressive and drop it since I don't have a functioning fuel system to drive it anyway. I just don't want to deal with draining it. But I may inspect to see if there is easy room to add a bulkhead fitting and a remove vent.

Mine cap I did blow out not knowing any better and air did escape when you applied enough pressure, so maybe that's the problem. Again, its hard to say when I cant troubleshoot since the pump isn't in it.

[TAQuest]

Originally Posted by RocktimusPryme

I have a repro tank which is probably based on a Camaro. I may get aggressive and drop it since I don't have a functioning fuel system to drive it anyway. I just don't want to deal with draining it. But I may inspect to see if there is easy room to add a bulkhead fitting and a remove vent.

Mine cap I did blow out not knowing any better and air did escape when you applied enough pressure, so maybe that's the problem. Again, its hard to say when I cant troubleshoot since the pump isn't in it.

If you had everything else right you could use a hose into a bucket and it would work good. Which actually is a good troubleshoot when you get your gauge. The only thing that goes wrong with tanks is they get plugged up with garbage if they are old or someone put something in there that wasn't supposed to be in there. Otherwise they aren't the usual suspect with fuel delivery problems.

[HWYSTR455]

Those Tanks Inc vents are junk, spray fuel all over the place, but technically, they work. Anything over a half a tank, if you slam on the brakes, it pours out the vent, and it does make the garage smell.

As Tom mentioned, 71-up got the fuel separator which is mounted in the trunk behind the rear seat upright. It doesn't care if it gets a big splash of fuel, it will drain right back into the tank. The charcoal canister is basically just to help remove vapor/smell. The canister also has a line that goes to a vacuum fitting on the carb, so heavy vapor gets sucked in and burned.

IMO the vented caps are not enough, certainly not for an electric pump/return system. You will suck the tank flat. The vented cap on a deadhead system I guess would be acceptable, but it is possible that pressurized fuel could get pushed past the mech pump. And, in a high HP app, the cap may not vent enough, causing a low fuel psi/lean condition.

Caps can go bad any time, and as you start creeping up on making more power, the venting can be a root cause of poor delivery. It may not be immediate, but you start dialing a combo in, and each step make 10-15 more hp for example, it will nose over at a point well beyond where it should be before you notice. For that and other reasons, I still say the 'true' solution is a return system, electric pump, and a properly vented tank.

Nice thing about when you plan a performance fuel system and install it, you never have to worry about it again (for the most part). No question if it's going to deliver enough, etc, you KNOW it's right.

.

[78w72]

Originally Posted by RocktimusPryme

But in order to do that you need a blank return line right? I was planning on using that port in the pickup when I get the 1100 pump back. Could you put a T in near the tank and mount a remote one like this higher up.

http://www.tanksinc.com/index.cfm/pa...prod/prd98.htm

I was actually looking to see if I could drill out the center of a cap and add one of those bulkhead style vents in the cap, but I don't think there is enough room.

that is the one i use, it works great for venting pressure & will shut off if a roll over happens.

no reason you cant run a T at the vent line & hook this up, thats where mine is at, then i ran it up the wall of the hump area in front of the gas tank so its higher than the fuel inlet. if you think you need to vent pressure then its worth a try. im curious what tom v says about pressure in the tank, if its a benefit then i dont think you need to mess with it, just deal with the slight pressure when its hot.

& by pressure in the tank i mean a minimal amount, not enough to force fuel past the carbs needle/seat. before i added the remote vent i would get some pressure when removing the cap, but never pushed past the needle/seat.

[78w72]

Originally Posted by HWYSTR455

Those Tanks Inc vents are junk, spray fuel all over the place, but technically, they work. Anything over a half a tank, if you slam on the brakes, it pours out the vent, and it does make the garage smell.

the tanks inc vent on my 2nd gen firebird works great... does not spray or slosh any fuel out the vent under hard braking at all. if you had one spray fuel, maybe it wasn't mounted high enough?

just stating my experience with it, i drive this car pretty hard on the street as well as hard stops at the track running low 11's @ 121+mph with at least 3/4 tank, sometimes full & never had any fuel leak or spray out of it.

[Formulajones]

Originally Posted by 78w72

the tanks inc vent on my 2nd gen firebird works great... does not spray or slosh any fuel out the vent under hard braking at all. if you had one spray fuel, maybe it wasn't mounted high enough?

just stating my experience with it, i drive this car pretty hard on the street as well as hard stops at the track running low 11's @ 121+mph with at least 3/4 tank, sometimes full & never had any fuel leak or spray out of it.

I think they have changed the design of it a few times now. The most recent one I'm doing now for someone does not look like the same one that came with my fathers Tank Inc setup several years ago. But I can tell you the one in my fathers setup acted just as Highway describes. It was terrible. It was mounted as high up as possible at the top of the hump above the rearend housing. There is no floor space higher than that. That thing was a leaking nightmare with anything over 3/4 tank.

So even though this new setup I'm doing on a 72 bird does look like a different vent design then previously, because of past experience I'm just not going to use it. I decided to modify the tank, added the necessary nipples to hook up the factory evap system so the tank will vent through the charcoal canister as intended. If I find it's necessary to add more venting I might experiment with the new style Tanks Inc vent valve and see how it does, but only as a last resort.

[78w72]

Originally Posted by Formulajones

I think they have changed the design of it a few times now. The most recent one I'm doing now for someone does not look like the same one that came with my fathers Tank Inc setup several years ago. But I can tell you the one in my fathers setup acted just as Highway describes. It was terrible. It was mounted as high up as possible at the top of the hump above the rearend housing. There is no floor space higher than that. That thing was a leaking nightmare with anything over 3/4 tank.

So even though this new setup I'm doing on a 72 bird does look like a different vent design then previously, because of past experience I'm just not going to use it. I decided to modify the tank, added the necessary nipples to hook up the factory evap system so the tank will vent through the charcoal canister as intended. If I find it's necessary to add more venting I might experiment with the new style Tanks Inc vent valve and see how it does, but only as a last resort.

that could be, i never saw any older ones, i bought mine about 2 years ago. its mounted just about where you said, but not all the way above the rear end, just on the higher point of the area as it goes up near the top of the rear end... at least 3-4" above the top of the filler neck.

on a 72 bird, they dont use a charcoal canister for the EEC, its just a metal box mounted behind the rear seat that doesn't have any charcoal in it. for whatever reason i eliminated that & went to a non vented 1970 tank, probably because it was a cleaner install without all the hoses going in & out. i saved it all though & may consider going back to a 1972 tank if i end up converting to a in tank pump down the road if i cant make my mechanical set up work better at the track.

[Formulajones]

Yep, 72 has what I call the charcoal canister, what is actually the plastic tank mounted on the core support with a vacuum switch on top that opens and closes the vent, as well as the metal canister you described behind the rear seat.

My 70 is kind of the opposite, it has the plastic charcoal canister tank with a vacuum switch, and a couple of extra lines that run back to the tank for venting, but it doesn't have the metal tank behind the seat.