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Vented Fuel Cap?


A TR7 16V
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I'm no expert, but. I have the original type petrol cap fitted and it does not a hole in the cap. I can't say I'm aware of how the system vents, sorry I can't be of more help.

Someone will be along soon with a full answer, my money is on either 'Doug' or 'Pete' being part of their name 😉

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well they have to breathe some how  there is  no seperate tank breather 

lockables breath through the key slot   on a std cap without having one to examine  i just dont remember  these will all be the dreaded 3 prong caps 

which makes alternatives a bit scarce 

Dougs GT6 has the pop up so different kettle of fish and he has to get his robot lawnmower going so do not disturb  ha !!!

Colin must have some to play with ???

Pete

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12 minutes ago, Pete Lewis said:

on a std cap without having one to examine  i just dont remember  these will all be the dreaded 3 prong caps 

I even went and looked at the one fitted to confirm there is no visible hole. How it vents I don't know, I agree locking ones would vent via the lock ( I have a locking one as well but prefer the original - less effort when I go and fill the car I'm that lazy)

Now my curiosity is aroused, I'll not sleep until I also have the answer!

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The problem seems to be that the pentangle shaped locking one I have has a seal round the barrel, so it didn't vent through the lock. There was some pressure in the tank when I came to take it off this morning, and a bit of a petrol smell round the car. I don't think the 5 sided shape is at the Rootes of the problem.

It was a tight seal on the neck of the filler, but I've taken it apart turned all the gubbins inside around, and now it's a bit slack. So I think that should ease the problem, but I'll keep an eye on it. It fits in what looks a better way, in that the cover for the keyhole points straight down  not down and back.

Graham

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1 hour ago, Pete Lewis said:

Colin must have some to play with ???

Pete

Original caps have no pin hole; they're just solid right across. I even took the label off one a while back to check - no pin hole. The locking versions may breathe to some small extent through the keyhole, as Pete says, but it doesn't appear to be a priority.

222901513_STcap.jpg.05575e339c87c0200d4a672083970d20.jpg  lockingcap.jpg.47c059f04920094534aabb3844a29b1f.jpg

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I got sold a US spec. non-vented cap for my first fake TR7 Sprint many years ago at the main dealers. It only took the fuel pump a day or two (doing 50 miles a day) to suck the fuel tank flat. I did notice it was a bit hard to get the bugger off, and it took only a dribble to fill the tank. So I looked underneath and saw it was a bit thin. Luckily I still had the receipt on the floor of the car and the parts man admitted he'd cocked-up. Got a new tank and hanger straps  from the dealers.

But with this non-vented cap on the herald, there was only a bit of pressure in there. I don't do so many miles and it was late morning when the sun had warmed the car up. But it's a surprise to me there's no vent anywhere.

Graham 

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2 hours ago, Colin Lindsay said:

Original caps have no pin hole; they're just solid right across. I even took the label off one a while back to check - no pin hole. 

That ties in with what I currently have fitted

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My Vitesse and Herald both died when i changed to a non vented lockable cap, remove the cap and a gush of air.I Modified them so they vented and no more problems,so i would say yes.

I have since gone electric pumps so i guess not relevant now.

 Steve

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Mid 90's daughters first Sprint I hated the zig zag exhaust system, which was in need of repair anyway so I had a larger straight front to back system fitted but the fitter accidently hit the nylon return pipe from the fuel tank (rear) to carbon Canister (front) with the oxy sealing it, so the tank couldn't breathe the resulting suction from the fuel pump collapsed the top of the tank in about 3in, obviously stopped the car in 1/2 mile, & only when the fuel cap was removed with a woosh did I realise what the issue was, the damage was permanent and the top of the tank didn't return so a new S/H tank was located, quite remarkable how much suction that little mechanical diaphragm fuel pump can create!

Exhaust fitter "no I didn't do that!" I fitted a separate breather, l also fitted a locking cap, which I assumed must breathe too.

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Snap, I was just looking for a photo for a different thread and also spotted ones of my original equipment type petrol cap and locking one. Same as yours, never noticed it before.

I will sleep much better tonight now the mystery is solved. 1302059103_SAM_1567B.thumb.JPG.0d644c97e084794893bdf9b0d743bf0e.JPG

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thats the answer 

 all tanks have to vent or atmospheric pressure will easily collapse the tank with a small vacuum inside developed from pumping fuel out 

does  not matter     what pump you use the tank has to vent or disaster is just down the road 

changes later in life to avoid fuel spill if the car turned over introduced non vented caps on many cars burt there then has to be a tubed vent system 

of your tank becomes a flat pack 

Pete

 

 

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1 hour ago, dougbgt6 said:

Is there something similar on the 13/60?

no the H and V models only have a breathing cap  there is no extra piped breather available unless you modify the tank yourself 

get on with the Ant -ics   Ha 

Im sure you remember the C4 Picasso  sucking the AdBlue tank flat on the way home from helping someone we should not have helped 

all doen to a non breathing cap fitted 

Pete

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There was no sign of the tank collapsing or fuel starvation with the cap as it was, just some pressure in the tank and a bit of a smell of petrol when it had been stood in the sun. The smell was mostly in or around the boot. 

I wonder then if the seal was acting as a one way valve, so that it allowed air in to replace the petrol being used, but would not allow vapour out as the day warmed and the vapour pressure increased. Looking at it, I'd have expected it the work the opposite way with pressure getting out as the cap was pushed away from the neck and vacuum building up as it was pulled tighter on to the seal. But it clearly wasn't sucking the tank flat. Maybe it's some other part of the system and pipework that was acting as a one way vent and that, perhaps, explains the smell.

I've been out now, and there's no smell, and obviously no pressure, with the cap rebuilt to not fit so well - I think it's which way around the lock bar is fitted determines the pressure between the filler neck and the rubber seal, but some of it could be which bit of the inside of the neck it bears on and how worn that is.  

I'm still gonna keep an eye on it, and watch for any slosh out the filler, etc. There's no sign of a bypass round the seal, but I guess I could cut one if I have to tighten the fit to keep the petrol in. It is a proper 3 pin fitting made by Wasso for the Herald/vitesse and Dolomites (it don't look so good on the doly though). So why it don't have a breather/vent is a bit of a mystery.

Graham

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petrol smells in a boot of hearld/vitesse is down to the seal on the suction tube which has a rubber olive to allow the reserve lever to swivel the suction pipe

when the tank is failry full slosh makes it leak  also the hose here goes hard and the jub clips dont seal , ends up you can also suck air from the hose leak and when you stop a small amount of fuel leaks out into the boot 

doesnt leak when standing as its all above fuel levels 

Pete

 

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As an aside, on the issue of the lock acting as a vent, that turned out to be a problem on the TR7 I had with a locking cap with the lock at the bottom of a small well. The TR7's cap is more or less flat on the deck behind the back window. So, whenever it rained, that well would fill, and the water in it would drain into the tank. Not much each time, but it accumulated. It wasn't a problem while I had SUs fitted. But when I went to Delortos, the steel bits in the accelerator pumps rusted, and the rust flaked off and kept blocking the slow running jets so it wouldn't idle. It took a while to work all this out though.

Graham

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7 hours ago, Pete Lewis said:

petrol smells in a boot of hearld/vitesse is down to the seal on the suction tube which has a rubber olive to allow the reserve lever to swivel the suction pipe

If petrol could leak out (or at least the vapour) That would be the manner by which the  air gets into the tank?. Even if the cap was fully sealed?.

Pete

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