How does the main fuel tank breathe on a ‘09 Def Puma?

[Robmacca]

Guys, I’m after some info on how the ‘09 Defender 110 Puma Main Fuel Tank breathes...
I’ve recently installed a sill tank and I’ve installed it in the way that it pumps fuel back into the main fuel tank as you’re driving along via turning on a switch on the dash.
My concern is that the main tank will not allow air to escape as the tank is filling, thus building up pressure to the point where the facet pump stops pumping fuel into the main tank.
This is what happened in my old Defender Tdi300 when I installed a sill tank on it as well...

So, can someone tell me how these tanks breathes because usually air escapes from the main filler when your filing up normally, but how does it escape when the filler cap is still on?

[martnH]

It's a tube on driver side and ends near the driver side rear wheel arch.
The breather has a "filter element" in the middle

I would say the breather should probably end higher than the silver tank otherwise tank would flow out of the breather?




p.s I replaced the factory tank with a lra long range tank....the lra tank comes with an additional breather on the passenger side, I appears.....

Cheers

[DiscoMick]

Mine has a breather coming out the top centre of the tank. Some have a breather running to the fuel filler hose.

[Robmacca]

So, this breather lets "AIR" - IN as the tank is drained(preventing a vacuum) and OUT as the Tank is filled (preventing pressure building) ??

[DiscoMick]

Does your sill tank fill through the main filler or thru a separate filler?
If it fills through a separate filler you may have to take the cap off the main tank filler to let air escape as the sill tank sends fuel into the main tank.
Or have I got that wrong?

BTW mine has a 120 litre tank with a breather running from the tank to the filler hose, so the air comes out as the fuel goes in.
Maybe if you research the Long Range Australia tanks there will be a diagram or picture of how it works?

[Robmacca]

Does your sill tank fill through the main filler or thru a separate filler?
If it fills through a separate filler you may have to take the cap off the main tank filler to let air escape as the sill tank sends fuel into the main tank.
Or have I got that wrong?

BTW mine has a 120 litre tank with a breather running from the tank to the filler hose, so the air comes out as the fuel goes in.
Maybe if you research the Long Range Australia tanks there will be a diagram or picture of how it works?

Because I've got the Inner Guard Fuel Tank there is a fuel line hose that goes from the bottom of the inner guard tank into the Main fuel tank via extra point that was put in when the inner guard tank was installed.

The hose that runs between the Inner guard tank and the main tank - I've simply Tee'd into that and this is where the facet pumps fuel into. I'm hoping this system will work but I do need to test it. I did find a breather that allows air in and out so it should be able to breathe. I guess I will find out as the Sill tank is now full and I plan to trial pumping it into an empty main tank as see how it all goes...

The Facet pump has a flow rate of 129ltrs/hr, so with the sill tank being 65ltrs, I'm thinking it should take about 30-45mins to pump all fuel into the main tank...

[roverrescue]

The main threaded cap breathes
Otherwise as you burnt fuel the tank would be restricted by pulling a vacuum
On the tdci they added a smaller 5mm breather line and small filter
This is just a leak point / allows water in
The main 3/4” breather ties to the top of the filler neck is all that is needed.

So you won’t have any problems , facet pump filling back to mainntank air will be expelled via fill cap

[Robmacca]

The main threaded cap breathes
Otherwise as you burnt fuel the tank would be restricted by pulling a vacuum
On the tdci they added a smaller 5mm breather line and small filter
This is just a leak point / allows water in
The main 3/4” breather ties to the top of the filler neck is all that is needed.

So you won’t have any problems , facet pump filling back to mainntank air will be expelled via fill cap

Well, not exactly.... I found the small 5mm breather line partially blocked and air was not venting out of the cap. As I was pumping fuel across I removed the cap and u could hear the air blow out the cap so it certainly wasn't breathing quick enough for the facet pump. I've since fixed the blockage on the small breather line and it all seems to be working but still not pumping across as fast as it should. It should take about 30-35mins to empty the sill tank into the main tank but it is taking longer but at least its working now...

[roverrescue]

Hmmm
Interesting
When I re-jogged the tanking in my 130 I Deleted that little
Line (was never on older models)
Ran new breather hose from lower tank to upper tank and then from upper tank to filler neck.
I have a 2” pipe joint both tanks, I can fill both 75L tanks at high flow with no venting issues and certainly don’t pull a vacuum burning that 150L

I wonder if your main 3/4” breather is restricted , especially considering the guard tank will have raised that breather???

[MLD]

I wonder if your main 3/4” breather is restricted , especially considering the guard tank will have raised that breather???

this is were i'd start. if the 20mm breather has fuel in it you will have the restriction. Trace it to see if there is any dip in the line that fuel can sit in and create a blockage. You can buy lengths of fuel grade hose from any truck supplier if you need to re-route the breather. If there is a blockage, the fuel going in will be limited by the capacity of air escaping, explaining your slow transfer time.

The other thing i've seen happen is the threaded bung to the main tank for the lead in line from guard tank is exposed to damage. If you don't already have one, a guard (bit of steel plate) is good insurance from a rock strike taking out your fuel supply. Seen it happen in the middle of the Flinders ranges. There were washing buckets, pots, pans and any other container we could get our hands on to save 127ltr of diesel.