Technical Fuel tank venting

[Starting a new and more detailed thread.]
29 Ford 302 with fuel tank mounted in trunk. Here and there, a few hours after filling the tank, gas will begin dripping out of the vent line onto the garage floor. This doesn't happen every time I fill up as I suspect it has something to do with extreme summer heat causing the fuel in the tank to expand and push out the vent line on a hot day on a freshly filled tank.

The fill neck is 3" tall. The vent line is basically flush with the tank thus 3" below the neck which I understand is not ideal. There is NOT a check valve/rollover. Just a hose line that runs out the top of the tank and down and through the trunk underneath the car with a filter on the end.

What are my options for a better venting setup? A check valve/rollover is supposed to be ABOVE the tank and vented outside correct? How can I do that without drilling a hole in the side of the car? I don't understand what to do at all. I'd like to still run the vent line through the bottom of the trunk under the tank. Is there any way to do this? What can I do? Would a shorter fill neck help so that the pump shuts off sooner automatically and the tank doesn't fill as much? Sorry for so many questions. Also here is a link to the tank: https://www.summitracing.com/parts/...Z6wDF4lt2tz6X5hUou9wnF6BFDID5D3BoCeAwQAvD_BwE

 

I suspect fuel slosh is filling the line. As soon as fuel gets past the first hump in the line it looks like it has nowhere to go but further down the line. You need to get it looped higher.

 

Any fluid will seek it's own level, so you need to raise the vent tube higher than the tank. Any gas you put into your tank that is higher then your vent tube is going to escape. That includes going around corners or up and down hills. It will also try to syphon any fuel it can until the syphon is broken, so if you park on a hill and the gas is higher than the vent it will do so. You can use the existing vent tube you have now so you do not need to drill any other holes. Why the filter on the end of the vent hose?

 

A loop in the line will help. Not depending on the pump nozzle shut off will help too. I have a similar setup with a roll over check valve in the vent line. I haven't had it push out any fuel, but I always make sure there is room at the top of the tank when I fill up.

 

I did mine a bit different in my roadster. I used a chrome vent that I bought from West Marine and drilled my rain gutter outside the weatherstrip on the trunk rail. That way the fumes would vent to the atmosphere and not in the car. Tank is behind the seat. the green striped hose is the vent.

This the vent in the rain channel, looking down from outside the car.

 

I agree with the above. In your pictures, you appear to have a lot of space above the tank. Run your vent line UP from the tank, loop is several times, then have it go down through the floor. I would use steel line for the looping as the rubber line may collapse from all the loops. I'd draw a picture but the nurses took my crayons away ... I kept eating them ... the crayons.

Nothing wrong with making the vent line a combination of steel and rubber (actually, by "steel" I mean NicCop as it is so much easier to work with than straight steel).

 

On older cars that used a vented tank had a line that ran a foot of more above the tank and looped back down

 

Put a loop in it, if you use rubber line you can zip tie the loop into place

 

What is the purpose of the loops? At work so it's hard to think straight haha

 

I always thought that if you loop the line like that and get a little fuel slosh, the fuel would settle in the loop and effectively block the vent. I don't know, that is just my thinking and like many things it may be backassward.

 

Thank you for the pic! What purpose does the loop serve? A few other people mentioned it, even adding more than one loop.

 

Can anyone clarify this?

 

The fuel pump will pull a vacuum on the tank and suck the fuel out of the loops in the vent line. Get your vent line up high, like up into the quarter panel, and then back down to vent outside the car.

 

Thank you! What purpose does a loop or loops serve?

 

Sloshing fuel is going to have to make one heck of a trip up and down, up and down (the loop or loops) in order for it to make its way out onto the ground. Since fuel is a liquid not a solid, it won't prevent the line from venting if some gets down into one of the loops. As the engine uses fuel, suction is created in the tank ... the weakest link yields to that suction, the weakest link (assuming your cap is not vented) being a bit of fuel in your looped line (as long as the end of the line that is not attached to the tank is open obviously).

Now ... GET BACK TO WORK !!!!!!

 

NHRA used to require the loop, I've never had issues with the loop and the car pulling the wheels every pass. I think the required roll over valve has superseded the loop, I assumed it was to contain fuel in case of roll over.

 

As a vent, don't put a loop in it. If it fills with gas, condensation or rust, it can't vent properly. A loop is required in a vibration situation.

 

As fast as gas evaporates it isn't going to sit in there even if it did make it to the loop.
You still run the vent line higher than the fuel level then make your loop before exiting out the floor, I've ran them for yrs, driving on the street, loading and unloading off an open trailer, doing wheelies every pass.
What's that thread, it works until someone that has never did it tells you it can't work lol

 

I have a similar situation to you, a '30A coupe with tank behind the seat. I had a steel vent line going straight up about a foot with a rollover valve on the top. Going around corners with a full tank resulted in fuel splashing out the top of the vent. I replaced the steel line with a piece of soft brake line and wound it like a vertical coil spring. About 5 coils resulting in a longer vent line that couldn't slosh out the top. It worked really well, no gas spilled, no gasoline smell, and I can fill the tank right up. Sorry, no picture

 

Fuel has inertia in a straight line just like anything else and can slosh a long way in a straight tube or hose. Newton reported that there's a law about that. (Rumor has it that it started with an apple falling but I know for a fact it was smacking on an upside-down ketchup bottle.) A loop is very effective at putting the brakes on "inertia".

 

That is total nonsense. The vacuum from the fuel pump wll pull air past it into the tank and pressure in the tank when it is sitting out in the heat will push it out when the air is released. What ever if anything that ever gets in the loop isn't going to cause any real restriction, It might blow bubbles if it pushes it all out but isn't going to restrict air going in or air going out. The fuel pump creates enough vacuum to collapse a gas tank that doesn't have much fuel in it if the tank isn't vented. Then throw in that full tank of cold gas will expand quite a bit if say you pull into town for a rod trot and fill the tank before going to the event and let the car sit out in the sun all day.

My truck was terrible about gas fumes at the back of the bed when I had a full tank like that. It's got a 20 gallon aluminum tank that really soaks up the heat off the pavement. It's got a hose running from the tank to the rear stake pocket and a piece of steel fuel line with a tight 90 at the end up several inches in the stake pocket.

 

On my roadster, I just made the hose longer and ran the hose towards the other side of the car and back and then it goes out the bottom as you did. Seems to work real good.

 

A loop is traditional.

 

No reason for loops .IMO

 

I've put a loop in the vent line in every car I ever built, and always done with hard line for the vent line. I route the end down outside the trunk area so any fumes wont be inside the trunk. I also put a small cheap $3 fuel filter over the end of the vent line just to keep critters from going in the line when it sits all winter in the garage.
I've never had any issues with fuel sloshing out, expansion, or smelly gas fumes.

 

I think we need a definition of loop , just line up & line down doesn't constitute a loop ,does it ?

 

The pump will pull air from a vented cap before it pulls through a small diameter vent , no ? A non vented cap it would have to pull through the vent tube .

 

Cause loops are cooler look'in than straight lines.......
6sally6

 

Put loops in it like the still has in the above picture, it takes way more fuel to reach the top, and the fuel runs right back to the tank when it cools back off leaving a clean empty line. Run it out the bottom, you don't want fuel vapors or liquids anywhere near your paint.

 

My idea of a "loop" is a 360 degree bend.