'85 110 - fuel tank not venting

[steveG]

I've got an issue with the fuel tank on my County not venting. When I stop to fuel up after a decent drive there's a definite suction on the tank cap, and I've noticed on occasions that I get a loud "donk" sound from the tank about an hour on the highway with a full tank. Presumably one of the flat panels on the tank oil-canning as it gets sucked in.

Its a long range tank, but everything connects back to the filler neck like a standard tank. I'm guessing that the venting must be meant to happen at the cap as there is no other way I can see for air to enter.Is that correct or am I missing something?
If it IS via the cap, any likely causes of it not working?

Steve

[Wallaby Ted]

My 85 County V8 did this eventually it will have trouble sucking fuel through if the pressure is great enough.

I found that the expansion tank that is mounted up behind the rear drivers wheel was blocked, after I pulled it out and cleaned all the mud out never had a problem with pressurisation since.


Richard

[flagg]

Do you have a spare fuel cap to try?

[Judo]

Interesting. I assume it hasn't always done it? It's funny how someone starts a thread for an "issue" which my vehicle has always done. I'm sure my Disco makes a good suction sound when I take the cap off. Always has... The cap itself could be key though as my cap has "unleaded" written on it, but it's a TDI, so obviously not the original. It has a 120L longer ranger tank.

I'm not sure about the side popping in and out though... I hear too many random noises to know if one of them is this.

Now I'm also interested to know if some suction is normal now!

Still, if it ain't broken, don't fix it. My Disco runs fine.

[steveG]

Interesting. I assume it hasn't always done it? It's funny how someone starts a thread for an "issue" which my vehicle has always done. I'm sure my Disco makes a good suction sound when I take the cap off. Always has... The cap itself could be key though as my cap has "unleaded" written on it, but it's a TDI, so obviously not the original. It has a 120L longer ranger tank.

I'm not sure about the side popping in and out though... I hear too many random noises to know if one of them is this.

Now I'm also interested to know if some suction is normal now!

Still, if it ain't broken, don't fix it. My Disco runs fine.

Interesting. My County is now a diesel, but was originally a V8. I definitely doesn't have any expansion tank as Richard described. I'm now wondering if there is a different arrangement for the diesel and if its perhaps been overlooked in the original conversion.

I'm not aware of it causing me any issues but it just doesn't feel right that the tank should end up with that much vacuum.

Looking in the later Defender parts manual there are 2 types of tank caps - vented and non-vented. I'm guessing the non-vented variety need to have a vent arrangement somewhere in the tank breather itself.

The plot thickens......

Steve

[schuy1]

Just have to be aware that some fuel caps are vented and other models are not. I have a '96 build Tdi and I had to get a new cap at 1 stage. I was sent a cap, fitted it and off I went, 300 kims later I got starvation symptoms! after some head scratching I found the cap was not vented as needed on that tank! It had suctioned on so hard I had trouble removing it and it had also oilcanned the bottom of the tank up! It still has a dent in it . pre '94 seems to be tank vented, post appears to be cap vented.
Cheers Scott

[Judo]

Interesting. Was that a Defender or Disco Scott?

Just did a few searches and it seems there is often 2 types of breathers/vents.

1. The filler tube incorporates a tank breather which allows air and fuel vapour displaced from the tank when rapidly filling to vent to atmosphere via the filler neck.
2. A breather within the tank that is used as fuel is used by engine. Small tube.

Obviously most vehicles have (1) but doesn't tell you one way or the other about (2).
If you don't have (2) then I would think you need a vented cap.

In the Disco TDI workshop manual it says.

Fuel system
A 89 litre fuel tank is mounted at the rear of the
vehicle beneath the load space floor. The tank is
vented by a 2 way valve in the filler cap.

Looks like I have the wrong cap at least...

Yours is a bit tricky with your 3 different engines Steve. If you can't find type (2) breather though, sounds like you need a new vented cap.

[tam242]

The cap on your county is a non vented type, as described earlier the original county vented through the over flow tank. To prevent the tank pressurising previously I had a breather hose that equalised the pressure that ran up higher than the tank.
Adrian

[steveG]

OK - all makes sense now.

Have just ordered a couple of motorbike tank one-way vent valves from ebay. Figure they'd do the venting job, and hopefully stop the fuel running out in the unfortunate event that we find ourselves dirty side up

Steve

[Brad110]

Didnt do it when it was in the Defender Steve.