Pressurized fuel tank?

[Cuzzo]

Do you have a breather line from the tank? If so where does this go if you dont have a charcoal cannister? Does it go to the inlet manifold?

[thechuckster]

and ideally, that line goes via a VSV valve that only opens on IGN+ power. You might get away with a cannister out of an EFI car as they usually have a one-way valve in the outlet-to-manifold port that allows the engine to scavange from that port but not pressurise the cannister.

[stradlater]

Hrm...

So I don't have a breather out of the tank other than the fat one that goes from the tank to the fuel filler inlet which I assume is for letting air out whilst it's filling.

So it appears that this might be the problem. I have looked at the top of my tank, and there's three 'outlets'. One feeds my surge tank (through a pump of course) and the other goes to the overflow from the surge tank. The third is capped off (by me). I'm assuming THIS might need to go somewhere.

Where should it go and/or what should I do with it?

Bearing in mind that this is all after market, so any stock charcoal canisters or fuel vacuum feeds are gone.

[Cuts]

somewhere higher then the fuel level, external to the cabin, with some sort of filter on the end.

[Cuzzo]

Uncap it and run it to a vsv then to the inlet manifold via a one-way valve.(dont use a windscreen washer one-way valve they dont last.

[stradlater]

I would assume that the fuel vapour will affect the mixtures variably though won't it?

[thechuckster]

has to go to a charcol cannister before the VSV - then to the manifold (cannister will also have a drain valve at the base). The cannister prevents volatiles that could condense out of the tank fumes from entering the manifold. There won't be enough vapour to alter AFRs.

[Cuzzo]

So then you wouldnt need a one way valve cause the cannister and the vsv is pretty much one?

[stradlater]

So, road legallities aside, does one need a charcoal cannister?

Clearly I need the tank to 'vent', but what's the most effecient and legal from a motor sporting point of view?

[thechuckster]

cuzzo: one way valve not essential but the stop valve in the canister won't have been designed with boost in mind.

strad: i have seen one rally car with the third vent simply exiting the rear of the car via a hose.

[stradlater]

I think I get it now - basically we need to get rid of the exhaust air/vapour, and putting it through the cannister and sucked back into the engine is the best way.

Guess I'll look into hooking something up!

[RobertoX]

You can stop it bubbling out of the tank without depresurising the tank.

The reason why it the fuel bubbles out is because the vapour (which collects in a pocket at the top of the tank) pushes its way out when the lid is taken off. If the only way is up though the filler tube and there is fuel in the way the petrol gets pushed out too.
If you add a seperate path from where the vapour collects (ie the top of the tank) to the top of the filler neck then there will be an easier path for the vapour to take.

Legalities aside, put a vent tube between the top of the tank and somewhere up on the filler neck (higher than the top of the tank) then there should be no more fuel spilling out when you open the cap. Provided that the vapour that collects is vented into the filler neck.

I'm not up with the regulations but I think you can't vent the petrol vapour to atmosphere, it's gotta run though the engine and burn off. So the most legal way would be to add a vent line to a charcoal canister and then to the manifold (with a one way valve if its a turbo like was mentioned).

[thechuckster]

(in addition to RobertoX's wise words)

which ever way you deal with it, you want the tank equalising internal pressure with atmospheric pressure. Ideally, the filler cap combined with the canister does this - with the cap acting a safety device only.

Being a highly volatile liquid, petrol will out-gas easily - even from being shaken up as one might get driving around (simple test: half fill a plastic oil container with petrol, tighten cap, vigourously shake, observe swelling of container, release cap to hear gas escaping). When you consider the large surface area of fuel in a tank and the constant stream of returning fuel from the FPR, the likelihood of vapour formation is high given all the physical activity of the liquid.

Note: the charcoal canister also catches moisture (in gaseous form) and causes fuel vapour that can return back to liquid form to be either caught and dumped via the drain, or sucked back to the tank when the engine turns off and the tank cools down (is why older cars have the VSV downstream of the canister). Ideally, the vent running to the motor only releases volatile compounds that cannot easily revert back to liquid form.

(sorry for the long speech, woke up way to early, need to exercise brain to stay awake).

[stradlater]

Well actually I'm not sure that the filler cap does do that. Because it doesn't vent unless the cap is off... It does have a breather hose like you say already, and this clearly isn't doing the trick.

[RobertoX]

Breather going to the correct place on the tank? Besides the surge tank, is it a factory installation? Or had the orientation been changed?

Blockage or kink in the hose?

And thanks thechuckster, but not wise, just simple common sense.

This used to happen on my corolla by the way. Only on a hot day with a tank more than 3/4 full, fuel would actually seep past the old cracked seal on the cap and dribble down the side of the car.
To stop it happening until I fixed the problem I used to rock the car side to side before opening the fuel cap. It would allow the vapor to collect in the filler and vent out with reduced bubbling.
Maybe give it a go till you find out what's happening