What Does A Fuel Cap Do?

[Duncan]

There must be some older, wiser heads here.....

what does the fuel cap do?

to save some posts, the correct answer is not "it seals the fuel tank"

[Trex]

it covers the shaft hole.

[Sinista32]

Seals in petrol tank fumes

Protects vehicle fuel supply

Pressurises the fuel tank

Protects the fuel tank inlet pipe

Prevents corrosion

[warps]

Duh stops fuel sloshing out of the tank on corners.

[MBS206]

Try to keep the condensation out

[admS15]

Stops jerks pissing in your fuel tank

[KiwiRS4T]

Short story:

How Do Vented Gas Caps Work?

By Steve Smith

eHow Contributor

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Vented Gas Cap Function

• The vented gas cap is designed to vent small amounts of air into the car gas tank line. The vented gas cap has a one-way release valve that is pressure-activated. When the pressure reaches a certain point (defined by so many pounds per square inch) the valve opens by a very small amount, relieving some of the pressure. In this case, the pressure is formed on the outside of the tank, due to the vacuum formed from the displacement of fuel on the inside. After the pressure inside the line equalizes, the pressure valve closes. The valve is set to balance the pressure with the surrounding atmosphere. Air is allowed in, but no fumes are allowed out of the gas cap, because of environmental concerns.

Inner Parts

• The vented gas cap has a chamber under the plastic cap handle that inserts into the fuel fill line. The cap screws shut, and this chamber fits relatively close inside the intake tube of the fuel line running to the tank. On each side of the chamber are small openings where air is able to enter. These chambers lead to the pressure valve, which, as the level in the tank decreases, allows air inside to remove any possibility of a vacuum forming.

How it All Works

• Because the fuel tank is pressure-driven from a fuel pump and gas naturally expands and evaporates, a large amount of pressure can build up inside the fuel system over time. This second kind of pressure is taken care of through the operation of the fuel pump mechanism, as the vented gas cap needs to be the one-way flap to keep evaporated gas fumes in.

The long story:

http://www.agcoauto.com/content/news/p2_articleid/151

d Links

[Victory]

All that I know from personal experience is that they are only supposed to vent one way and that it is important that they vent correctly.

• I had one in a Subaru stop venting and the steel fuel tank started to collapse because of the level of vacuum being created.

• In our R33 track car the cap started venting both ways which resulted in fuel covering the rear of the car - not a great look when you have the usual flames from the exhaust

[Duncan]

thanks gents that is exactly the part I was trying to get to. funnily enough it struck me that the workshop manual might have some information, and voila!

so yes, the GTR cap has a one way pressure sensitive valve that allows air in to replace the used fuel (but not out, of course)

the other half of the breathing is the "fuel overflow prevention system with check valve" which breathes excess pressure into the charcoal canister when the engine runs.

the motorsport question of course is what is needed to safely vent the tank, I don't want to sit on a fuel bomb. Pretty much every aftermarket vent is an anti roll over valve which is open at all other times, ie it allows air/fuel vapour in or out unless the car is exactly upside down.

ideally I would keep the stock cap and have a 1 way valve to allow fuel vapour to vent to atmo (via a small filter) but I am having trouble finding something good. most setups are small valves near the top of the tank that are almost guaranteed to leak fuel from a full tank in high G corners. In fact the most common valve mounts on top of the tank and doesnt allow for any venting hose at all

Some people combat that by having looped breather hose but as soon as you get liquid fuel into that hose it can no longer work as a vent which defeats the whole purpose. kind of like a S bend with the same result that gas cannot pass.

happy to hear any suggestions about a good way to deal with the venting

[Neil]

Here ya go buddy,,,fuel breathers and roll over valves. My god some are stupid money.

http://www.demon-tweeks.co.uk/motorsport/fuel-tank-vent-valves

[fatz]

In the case of a import skyline

f**k all

[niZmO_Man]

In the case of a import skyline

f**k all

At least Skylines don't throw error codes if the fuel cap isn't on tightly.

[Snowman]

We had a failed breather on our EVO at the Winton 300. Caused the fuel bladder to expand, pushed the carbon bucket down onto the drive shaft - and wore a hole through the middle of it.

Thankfully we didn't a) blow up (it's right next to the exhaust) or even b) run out of fuel at the end.

I'm still not 100% sure how either of those didn't happen however.

So with the new fuel bladder we now have 4 breathers rather than 1.

[Duncan]

lucky outcome really....

I was not happy with the design of some of those breathers they are very basic. Have you got a pic of how the 4 breathers are mounted and vented? Are you using a 90L FIA with siamese fittings?

[Snowman]

I actually haven't seen it and it was fitted before I got the chance sorry..

It was your new mate Dylan Thomas who did the 4 vent design. He had previously suffered the same thing we had with the base Ralliart design so they did a 4 breather design. I actually just purchased Dylan's old tank as they were going from the saddle design to a boot mounted flat design as he was being team serious and wanting to extract every last litre out of his tank for 6hr .

I personally am not a fan of the safety aspects of a boot mounted tank though but that's a different topic to this one.

But Dylan would be a good bloke to chat to about fuel tank breathers.

[Roy]

And who was the muppet on the re-fuelling rig?