Fuel tank venting

[qkingston]

Before I put the body on for the final(!!!) time, I was checking the tank venting arrangements. I happened to read in "Kit Car" magazine (May 07) that "The Fuel filler must vent the tank to avoid pressure build up..." Question is, if I have a fuel tank vent fitted to the top surface of the tank, do I also need a venting arrangement from the filler (non venting aero cap) ?

Many thanks

QK

[lutman]

You do not say if the top of tank vent is a one way valve, or how it vents to atmosphere?
It should allow release of excess air pressure but not permit fuel leakage.

Usually what happens is there is a vent pipe linking from the top of the tank with a pipe that is just on the lower side of the filler cap pipe going into the filler line.That way any excess air pressure during fuelling vents out through the open filler cap - which must be open to permit fuelling !!
If you do not have such a vent you will get lots of fuel filling shut offs and blowbacks out of the filler neck.

[qkingston]

Thanks for that; the top of tank vent will have a one-way valve in the line (it doesn't yet - any suggestions for a supplier?) What I could do is use the non-required return line connection (carb fitted) as the vent path between the tank and filler pipe ??? That sound ok?

Rgds QK

[lutman]

Cannot advise a supplier for non return valve as I did not fit one.
Easiest way I know of is to hook up a pipe, can be same sort of size as a brake line, from a fitting at the top of the fuel tank which then goes back to the top of the filler neck, but opens out into the filler neck.That way excess air pressure is vented but any fuel that goes up the vent line returns into the tank back down the filler neck. Once the fuel cap is closed it seals the tank and the vent line. The vent line is purely to allow air to bypass the fuel as it enters the tank other wise you get an air lock and this in turn can cause the fuel to be blown back up the filler neck and all over your hands and legs.

[craggle]

If you have a non vented aero type filler cap though, You will need to vent the tank to atmosphere somewhere outside the vehicle.

If you have a fitting on the top of the tank I would run a pipe from this, up as high as possible under the body then out one wheel arch and back down to the chassis rails. No valves are required.

The Dax method is to have a pipe run from the side of the filler tube across the vehicle and out the wheel arch on the opposite side of the vehicle. The pipe supplied for this is simple window washer tubing.

Craig.

[qkingston]

Thanks guys; forgive me for being slow here but as far as I can see there 2 different issues. One is the venting of the tank to atmosphere to prevent pressure build up from vapours the second is the venting of the tank during filling to allow pressure to equalise and prevent blow-back. The single tube from the tank to the filler pipe will achieve the second objective, but not the first when the cap is locked shut. The vent path from the tank to atmosphere achieves the first objective, but possibly not the second. So do you need both methods ???

VMT
QK

[craggle]

I only had the vent to atmosphere on my car with the aero type cap. I now have no breathers as my Aston type filler cap is vented itself.

You do have to fill it a bit slower than normal though. About half way on the petrol pump gun is enough or else it fills the pipe up and spits back.
Maybe a vent back to the top of the filler neck would stop this? Not sure as I don't have the fitting on the top of the tank to try it out.

You definitely need the vent to atmosphere though, to allow air back into the tank as the fuel is removed whilst driving. Failure to have a vent can result in vacuum in the tank causing fuel starvation or even distorting the tank.

Craig.

[davidcot]

I have fitted an ASTON cap. The internal lockable fuel cap iHas either a Red or Green internal coloured plastic. Green is a vented type. Unfortunatley I had a RED one . Simple fix was to drill a small hole through the cap to the side of the lock using a 1/16'' drill bit, to allow venting.

I have had no issues with the tank squeezing of fuel starvation. So I have not tinkered with vent pipes.

Pics of cap & insert attached. The red insert can just be made out in the background.

DC

[Stu J]

Originally Posted by davidcot

I have fitted an ASTON cap. The internal lockable fuel cap iHas either a Red or Green internal coloured plastic. Green is a vented type. Unfortunatley I had a RED one . Simple fix was to drill a small hole through the cap to the side of the lock using a 1/16'' drill bit, to allow venting.

I have had no issues with the tank squeezing of fuel starvation. So I have not tinkered with vent pipes.

Pics of cap & insert attached. The red insert can just be made out in the background.

DC

Can anyone post give a clearer indication of where to drill the hole in the lockable fuel cap like Davids, I have the same issue with his.
I have the red lock too and had pressure build up from fuel when filling up earlier, so my curiosity is answered although I already thought it was a vented lock.

[Simon R]

Originally Posted by Stu J

Can anyone post give a clearer indication of where to drill the hole in the lockable fuel cap like Davids, I have the same issue with his.
I have the red lock too and had pressure build up from fuel when filling up earlier, so my curiosity is answered although I already thought it was a vented lock.

Hmmmm ... Just to add to the confusion... I have a red lock that IS definetely vented. Have they changed recently?

Cheers,

Simon