Fuel Filler Cap - to vent or not to vent....

[Matt Elan]

So after faffing around with the horn I took a quick look at at my Plus 2s filler cap. And the reason I sometimes get fuel leaking on the rear wing was apparent - it's a vented cap. Probably from a Norton Commando. It should be unvented, and the chrome is poor anyway so I tried to order a new cap refurb kit from the supplier, Ceadees of Wolverhampton but they appear to be closed.
Anyway heres a shot of the underside of the cap with the vent hole obvious at 1 o'clock.... Spot of araldite should fix it while I wait for a new cap....

[DJW]

Do the MX5 mod, worked a treat for me viewtopic.php?f=40&t=31759&p=331224#p331224

[alan.barker]

+1 the only way is the MX5 mod.
You can then buy a locking Fuel Cap from sjsportscars.
Lots of past post on subject.
I've even got owners of classic TVRs adopting the Lotus +2 MX5 mod ☺
Alan
Fio the biggest leak is in the middle. There is a second Seal like a Rubber Tube underneath which is useless.

[alanr]

All very well saying do the MX5 Mod but I cannot find one to buy anywhere, unless someone knows better? I have had one on order from MX5Parts since last September.

Alan.

[alan.barker]

Maybe you could order from USA.
Or you maybe lucky at a car breakers then you can get the Filler Neck also.
Untill then you need to run on 1/2 tank.
Alan

[alanr]

Thanks for the ideas but I have had, so far, no luck in finding one.
It is no big deal really and I have done the normal Club Lotus fuel tank breather mod to eliminate the garage fuel smell issue and with new cap seal I have no problem running with 1/2 to 3/4 tank. I just have to be careful not to get carried away when re-fuelling.

Alan.

[DJW]

All very well saying do the MX5 Mod but I cannot find one to buy anywhere, unless someone knows better? I have had one on order from MX5Parts since last September.

Alan.

Same here and cancelled mine after 6 months last month. Just buy a second hand fuel filler enquiring first if it has valve fitted. Mine did

Something like this https://www.ebay.co.uk/itm/MAZDA-MX5-EU ... 2920694139

Or https://www.ebay.co.uk/itm/MAZDA-MX5-EU ... 2661630032

Or https://www.ebay.co.uk/itm/MAZDA-MX5-MK ... 4223782941

[alan.barker]

There's also a supplier in Germany
Alan

[alanr]

Thanks for the ideas folks....
I will investigate.

Alan.

[Matt Elan]

The point is that there should not be a vent hole in the cap. Without the vent hole and if the three seals in the cap and the rubber elbow are in good shape then fuel will not leak out of the cap assembly so there is no need to go down the MX5 / Federal Plus 2 mod anyway.

[alan.barker]

Hi Matt,
what you are saying makes sense but in practice the only way is the MX5 mod.
There are 2 Seals on the Cap.
One is the large dia Disc.
The other is under the Disc and is a Rubber Tube which needs to compress.Every time you open and close the Fuel Cap. How this Rubber Tube can compress while closing the Cap then expand after the Cap is closed to effect a good Fuel tight seal i don't know.
If you don't do the MX5 mod the only way is to have a fuel tight Plug to slide inside the Filler Neck. The problem with a Plug is it will be a problem if you have done the Breather mod from top ofFuelvTank.
Alan

[DavidLB]

i have a brand new one that you could have. I have done the mod and it works David

[Matt Elan]

Hi Alan
I take your point but I think the MX5 fix is not a very neat solution - we should be stopping the fuel escaping from the cap rather than stopping it getting that far .
I also suspect that the seals are probably not as resistant to ethanol as they might be - especially old ones which will cause problems. However, I'll put an unvented cap on mine and report back once our lockdown is over (or maybe I'll nip out for a food shop in the beast!

Incidentally there are three seals in the cap if its a locking one - there's an extra ring seal around the lock to give the fuel somewhere else to escape....

[mbell]

My theory is that the problem isn't so much that the filler cap leak, it's more that the filler cap are being lifted by the weight/force/pressure of the fuel when Corning. Allowing some fuel to leak out.

The mx5 mood works by peventing that force being transmitted to the filler cap, there for the cap stays seated and no fuel leaks.

I plan to do it to my car at some point but don't keep much fuel in it so not an issue for my usage and there fore not a major priority.

[DJW]

In my humble opinion I have no problem applying modern design / tech, where hidden away, to improve on an existing design. Especially when fuel is involved.

Each to their own though.