Hi all
Im finally getting to the stage where the car is showing signs of life turned it over from the ignition last weekend. Im fitting the fuel system at the moment I have fitted the tank and filler cap. flow and return lines and pump will be going in at the weekend. The question is dose the tank need a breather to release the vacuum produced as the fuel is used? The engine is a mk5 1600 cvh efi xr2i/3i I seem to remember that air was sucked into the tank when I used to fill up the escort I think ( was 2 1/2years ago now) which would indicate a sealed system but that was with a plastic tank would the vacuumed be able to crush the ally tank? Or make the pump work harder?
Thanks Nick

[Edited on 22/2/07 by westcost1]

I would have thought a breather would be neccessary

- however I have never understood how cars which give a whoosh of air into the tank when you take the filler cap off work any way as the tank must be operating as a vacuum which must make it hard for the pump to get fuel out.

So I dunno maybe someone who really knows will tell us

Cheers

mark

Yup.

The Fuel out of tank needs to be replaced by somthing.

regards

Agriv8

wrap the breather pipe right around the outside off the tank, so that if the car ends up inverted, fuel will not pour out the pipe

Definitely needs a breather in the top, preferably with a non-return valve fitted

I've a Mocal one way vent.

If you do vent it down the side of the tank, don't take the pipe too low

Ok thanks I have had a look at the fuel tank breather valves on dimon tweeks how do they fit to the tank is it welded on or bolt in? and what dose it mean by medium high and max flow?

i have fitted a Locking cap

and no breather (still have on in case i need one- a red one from mocal-)

Anyway i guess air is sucked in by the hole in the cap as far as i know the woosh is over pressure not untherpressure in a cars sytem..

Tks

If your fuel system is airtight without a breather it will gradually implode as the fuel is pumped out. You need a breather to prevent this but most of the best ones are two-way valves which will allow air in and out but not fuel out, using two ball bearings of different weights (not quite sure how they work) This allows for expansion of the air on a hot day but still prevents fuel draining out in an 'upside down' incident.
The simplest ones (which I used) are often used on Motocross bikes. These let air in easily but also a small amount out (for expansion) Fuel is still prevented from flowing out as the exit path is very small.

You can fit in-line jobbies but ideally you need to weld a stub on top of the tank. Alternatively you could put a stub into the filler neck or cap.

Thanks raz man didn’t think about expansion on hot days. The tank has a pipe for a breather on top but this means i will have to put an extra one on for the return line.
Found this on how stuff works could explain the rush of air.
Fuel tank vent
Since the advent of emission controls, tanks are equipped with a control system to prevent fuel vapor from being discharged into the atmosphere. A vent line in the tank is connected to an activated carbon or charcoal filled canister in the engine compartment. Vapors from the tank are stored in this canister, until they can be purged later for combustion in the engine. On many carbureted engines, the float bowl is also vented to this canister.
Rescued attachment 191.jpg

thanks fesycresy who has pointed me in the direction of www.thinkauto.com for the valve lots of good stuff there