Workshop Notice 371 - Fuel Tank Breather - Which way around?

[revilla]

My car hasn't had the fuel tank breather one way valve modification done, Was going to do it tonight. Bought a Sytec one way valve off the internet rather than paying Caterham prices.

But now I'm confused ...

What is the purpose of this valve?

• To allow air to enter the tank as the fuel is consumed, preventing a vacuum damaging or collapsing the tank, in which case it should be connected to allow flow inwards towards the tank from the outside vent only.
• To let excess vapour pressure in the tank escape, in which case it should be connected to allow flow outwards away from the tank to the outside vent only. In which case, what allows air to enter the tank as the fuel is consumed?

Thanks,

Andrew.

[SM25T]

Air in ! Otherwise fuel will leak out on corners .... like it did with the open hose.

[revilla]

Except the valve I have requires quite a pressure to open, so in the absence of pressure would be closed to fuel slopping out. But that is how I originally understood it, however:

Quoting from WS371:

Push a 500mm length of 5/16" fuel hose onto the tank side of a fuel vent valve...

push the open end of the 500mm hose onto the fuel neck filler breather line ...

 

On the canister side of the vent valve push on a 370mm length ...

The tail of the 370mm hose should now just reside out of the bottom of the chassis.

Now every diagram I can fine for one way fuel breather valves on the internet shows them allowing flow FROM the tank TO the charcoal canister (which then absorbs the escaping fumes), which when read in conjunction with the above means the valve will allow flow OUTWARDS, contradicting my understanding and what you have just said.

Hence my confusion!

[Ian B]

The mod is for cars without a charcoal canister:

To minimise fuel loss through the breather on cars not fitted with charcoal canisters
(UK spec and race cars) under hard left hand cornering, the following system must be
fitted.

The valve should allow air into the tank, and not fuel out 

[ECR]

Simple valves just allow a one way flow so it allows air in and prevents fuel flowing out. The more sophisticated valves (and I think Sytec is one) allow air into the tank whilst preventing fuel flowing out and at the same time allow any build up of air pressure in the tank (perhaps due to high ambient temperatures) to escape. Sounds clever and probably requires mounting vertically .....

[revilla]

Thanks, I've fitted it so that if I blow hard up the dangling breather I can blow into the tank, so allowing air in and no fuel out. The arrow on the body of the valve points towards the tank. Thanks for all your responses.