Gas (petrol) caps: vented vs. non-vented

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Geoffrey Bard - 31 Jul 2004 00:44 GMT

Hi all,

When I imported my Mini last year, it came with a locking gas cap...which I
discovered even the best lock shop could not duplicate the key for. Not
needing a locking cap, and not wanting to lose the only key and get locked
out of my gas tank, I bought a nice cap (a Stant 11623) from a local auto
parts store that fits just fine.

Problem is, when I take a hard right turn after filling the tank, gas
dribbles out...not good. Thus my introduction to the difference between
vented and non-vented caps. The locking cap was non-vented, the new one is
vented.

I'm foggy on why they make both kinds. Can anyone enlighten me? As I
understand it, if the gas tank is vented, a non-vented cap is fine.

I don't think my mid-70's gas tank is vented, so wouldn't I need a vented
cap? Is there any harm in using a non-vented one? Finally, can anyone
suggest a non-mail-order source in U.S. for the right cap, whatever it might
be?

Thanks in advance!

Geoff (in Portland, Oregon, USA)

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k - 31 Jul 2004 01:32 GMT

Hi,
The tank should be vented. There is a pipe at the top of the tank inside the
boot that leads out under the floor, therefore you fit a non-vented cap.

Keith

> Hi all,
>
[quoted text clipped - 20 lines]
>
> Geoff (in Portland, Oregon, USA)

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Geoffrey Bard - 31 Jul 2004 19:46 GMT

Thanks, Keith - never really looked at the gas tank much, and I did find the
vent tubing leading from the top down through the boot...so I'll look for a
non-locking non-vented cap!

Geoff

> Hi,
> The tank should be vented. There is a pipe at the top of the tank inside the
> boot that leads out under the floor, therefore you fit a non-vented cap.
>
> Keith

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Graham W - 01 Aug 2004 04:43 GMT

> Problem is, when I take a hard right turn after filling the tank, gas
> dribbles out...not good. Thus my introduction to the difference between
> vented and non-vented caps. The locking cap was non-vented, the new one is
> vented.

Mini petrol caps, even the supposedly sealed type, are notorious for
leaking on bends and parked on slopes.

> I'm foggy on why they make both kinds. Can anyone enlighten me? As I
> understand it, if the gas tank is vented, a non-vented cap is fine.

Older cars had vented caps so that the air could get into the tank to
replace the fuel you use. The tank only had two holes, the filler and
the line out. In hot weather these cars leak hydrocarbon fumes from the
fuel cap.

Newer cars have vapour recovery systems which recover the fuel vapours
in the tank. Instead of a vented cap, they allow air and vapour movement
into and out of the tank via a charcoal canister, etc. The plumbing is
quite complicated. If you fit a vented cap, it effectively introduces a
vacuum leak into the system.

> I don't think my mid-70's gas tank is vented, so wouldn't I need a vented
> cap? Is there any harm in using a non-vented one?

If you use a non vented cap on a sealed tank (like when I recently
replaced the vented tank cap on my 1984 BMW motorcycle with a sealed cap
from a Camry), then no air can get in.

In the case of my gravity fed bike, after perhaps 15 minutes I had drawn
enough fuel off the tank to generate enough vacuum to stop any more fuel
flowing. Bike mysteriously stops. I check carbs - empty! I check tank -
plenty! I try starting bike - no problem. I just let the air in while
checking the fuel level!

Eventually I cottoned on to what was going on and punctured the seal in
the cap. All was well again.

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Geoffrey Bard - 01 Aug 2004 22:29 GMT

Thanks, Graham, for your useful comments...now I just need to locate a
nice-looking non-vented cap!

Geoff

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