Fuel filler question

Does anybody know if the standard fuel filler cap assembly allows the tank to breath??

I have heard that it does, but don’t know for sure. I have an after markest fuel filler fitted that doesn’t allow the tank to breath.



I know that when I undo the cap I hear a rush of air. I presume that this means that the tank is not at normal pressure/allowed to breathe.

There must be some sort of pressure relief valve fitted but that’s not what you are asking about is it?


I was hoping that the standard fuel filler cap doubled as the breather as well.

The reason I am asking is that my new tank is similiar to the standard one and has the large and small filler pipes at the tank end that are routed into one at the filler neck. I guess they do this to allow air out when you are filling the tank, so you don’t get covered in petrol as you fill up. My bag tank doesn’t have any other breather so I have to make the smaller pipe the breather and if the standrd filler cap doubled as a breather it would be job done. As it doesn’t seem to I will have to think of something else.


Just had a look at a cap I happen to have on my desk and it looks like it has a valve in it.

Of course it does!
The hiss you hear when you open the tank means vapour escaping. The valve only would let air IN, not out.
As you use fuel air has to come in from somewhere.

I looked at this in the past as well, and decided to keep the original BECAUSE it has a valve.
BTW, hat’s one reason it’s so heavy.

Don’t forget the standard car breathes through the roll over valve and charcoal canister, so that is why I asked the question.

I am undecided wether to simply revert to a standard filler cap, or put in a proper breather with seperate rollover valve and stuff.



Yeah, it would certainly be more simple to do. The only tricky bit will be getting my hands on the standard stuff at a decent 2nd hand rate.


And… where’s yours then?

My car had an after market filler on it when I got it.


Hasn’t the aftermarket filler got a vent/valve on it?
I’d be surprised if it didn’t as it could cause all sorts of problems for the fuel system.

My aftermarket version has a valve in it, works fine and is a hell of a lot lighter … but I have the orginal pipes and stuff but not the filler cap ( ?) if you need anything.

Hasn’t the aftermarket filler got a vent/valve on it?
I’d be surprised if it didn’t as it could cause all sorts of problems for the fuel system.

I know, I thought it was weird. But it does suck air in when you take the cap off.

Mine does still breath through the small one roll over valve pipe, but it isn’t ideal. I want to get my new setup breathing properly

Cheers Andy!!! Will see how i get on.


Sean, this may be of some use


the replacement part for the innards of the fuel filler cap has been located by Ian Bath, so well done to him!
The correct inner cap is the Stant part #10501 Pre-release locking fuel cap.

Now, Google come up with a few places but RockAuto were the most positive about shipping abroad.

In Ian’s own words: “The cap fits the S1 Elise and the ally cover and it has identical keys and mechanism to the original. All you would have to do is strip out the old knackered inner and glue the new one in place. I opted for the cheapest delivery method and it came in about 10 days. Total cost was $35.42 (USD) and there may be import duty implications but I didn’t get any.”

So there we go, for about $35 (USD), currently that’s about ?20 (GBP) you get a new fuel filler cap with the new key!

All the info here is from the BCBBS thread found here: http://www.british-cars.co.uk/cgi-bin/gen5?runprog=lotusbbs&mode=thread&access=&subject=2050&source=T&thread=2005061018080412989

Cheers matey, still undecided what to do.