1995 110 Defender vented filler cap?

[Lumpy Norman]

Dear all,

Ever since I've owned this vechicle (about six months), there's been a noticeable pressure difference between the fuel tank and the outside world when filling up. Removal of the cap with an empty tank results in a definite in-rush of air and an audible "fwoosh".

I'd always thought of this as being a good sign that the tank doesn't have any holes in it, but I've recently been informed by a reliable source that this shouldn't happen at all and that at no point should a positive or negative pressure build up in the tank.

The filler cap "looks" proper and is unlocked by the oldest looking key on the chain (for what evidence that's worth at any rate).

My question then, is should this cap be vented? There's a metal plate on the tank-side of the cap which is held on with two screws by way of identification. Is it possible to check whether the venting system works? I've seen mention of a breather hose on fuel tanks too, is this correct? If so, where might I find the outlet?

All help gratefully received,

Steve

[mmgemini]

IIRC there are two filler caps. A grey one, non vented

A black one vented BUT I bought a new vented black cap that wasn't vented.

I took the cap to bits and drilled a 1.5mm hole somewhere, sorry I can't remember where.

Note that you will need a new barrel and key as nobody seems to be able to remove the barrel from the cap

HTH

[Jode]

The caps were AFAIK vented as standard on the 300 TDI. It isn't unusual despite the vent to get a whoosh when taking the filler cap off as the fuel tends to expand as it gets warn during running the vehicle, and the vent doesn't completely equalise the pressure in the tank with the atmospheric pressure.

You can disassemble the vented cap to service it - I'd attach a pdf of a service article on how to do this if I could find an attachment tool on this website, however I can't seem to locate one.... but here's the link to the article:

http://www.landyzone.co.uk/lz/f41/fuel-cap-72130.html

[western]

you can use the more reply options button to upload pdf straight from your PC, it works for me.

[Lumpy Norman]

Don't know whether posting anything to this old thread will bring it back to life or not, but the problem seems to remain, but with differences:

I bought a new filler cap in the end which seems to have fixed the problem of the sudden pressure equalisation, but is has shown that the tank is still being pressurised as diesel is evidently being blown out of the filler cap while driving.

I've seen a thread elsewhere about a similar problem on a 2.5TD but, as mine's a 300tdi and recently had a full fuel injection pump overhaul and refurbish, I would like to think the cause isn't to do with the pump.

Does anyone have any idea of what might be causing pressure to find its way to the tank and, if so, how to fix it? The problem's been noticeable since I've had the vehicle and a fuel injection pump service hasn't made it go away.

The truck pulls like a train, doesn't smoke and returns about 30mpg, so indications are good that nothing terminal's wrong, but I would like it fixed since (a) fuel's expensive enough without spraying it out unused and (b) I'm followed everywhere by the smell of diesel.

All help gratefully received.

[CwazyWabbit]

In your first post you said the tank had a vacuum (definite in rush of air) now you believe it is pressurising and blowing the diesel out.

Where exactly is the diesel blowing out? The key hole? Around the cap past the seal?

Do you still have the old cap? If so Does it leak diesel at all with that on or does it just build up a vacuum/pressure?

Is there a chance the new cap is faulty and just leaking at the seal?

[jbs]

That would definitely say to me the cap seal wasn't doing what It's supposed to do-Seal! have you by any chance got access to another fuel cap and see if that makes any difference-perhaps borrow one from somewhere?

John

[Lumpy Norman]

With the benefit of hindsight and more evidence since my original post, I would say that:

(a) The pressure is positive and not negative. Originally I had assumed the pressure to be negative, since removal of fuel without letting air in would cause negative pressure. However, the presence of diesel around the filler spout would suggest that it's being forced out somehow.

(b) The presence of diesel around the spout has only become apparent since putting the new cap on. However, the rush of air and accompanying "thunk" noise as the tank re-shapes has stopped. This suggests to me that the old cap was blocked and the new one is indeed venting properly.

© It's not clear where the diesel's coming out, sort of everywhere and nowhere around the cap really. The keyhole does look a bit diesely, but the neck of the spout is where most of the "flow" is.

(d) I don't think the cap's the problem anymore, more the fact that the tank is receiving excessive pressure from somewhere. That's what I'd really like to fix.

Cheers,

[CwazyWabbit]

If you think about the fuel path there are very few places for positive pressure to enter the system.

The fuel pump, injector pump and injectors possibly. If any of those were faulty then I'd have thought your engine wouldn't be running as well as you say.

[reb78]

Are you sure its blowing out and not just leaking when you go round corners and the like?

A lot of the time, this sort of thing is either a faulty seal on the cap or a leak where the top of the filler neck is brazed to the top section that the cap attaches to. Replacing one or both often solves the problem.

[Jode]

Another possiblity is the filler hose from the neck to the tank. Is the rubber still in good nick or are there cracks? Of course this wouldn't be the source of diesel on the bodywork around the filler point, if that's what you're experiencing.

I happened to be going through the Bosch VE booklet, and it says the following about the fuel circuit from the tank:

"The fuel tank must be of noncorroding material, and must remain free of leaks at double the operating pressure and in any case at 0.3 bar. Suitable openings or safety valves must be provided, or similar measures taken, in order to permit excess pressure to escape of its own accord. Fuel must not leak past the filler cap or through pressure- compensation devices. This applies when the vehicle is subjected to minor mechanical shocks, as well as when cornering, and when standing or driving on an incline."

Pressurisation in the tank could only be caused by the injection pump. The lift pump has a single line that runs to the injection pump only, whilst the injection pump has both a fuel return line plus one supply line for each injector, with the injectors having their own shared fuel return line. What the injectors don't use is pressurised by the injection pump, hence really the injection pump is the only source of pressurised fuel returning to the tank.

I second reb78's ideas regarding the likely problem - cap sealing.