A little pressure or no pressure - that is the question...

Hi all,

Im seeking clarification on what seems to be a strongly divided topic!

The 300TDI fuel fee system....

The facts as i know them

1) The cap is vented, but only in a vacuum environment, ie, it can only suck air in to replace the diesel that which has been drawn - not allow any pressure build up in the tank to vent out

2) The tank breather reconnects to the fuel fill spout pipe _before_ the cap, not into free-air like some of the older models did - thus making the whole tank area somewhat air-tight

3) This is how they came from the factory!

So based on the three facts above, we can deduce somewhat that the designers expected to have a little pressure build up in the tank, from when the diesel expands on a hot day for example....

I appreciate that a faulty turbo diaphragm could also pressurise the tank a lot - which again shows that the tanks are air-tight, but shows a more violent form of pressurising.

Now, let me explain the problem i had over the last 2 weeks

Woke up one lovely sunny morning, (after filling up the day before) to see diesel dripping onto my drive, never seen this before, upon inspection, it was dripping from both the fuel sender seal and drain plug....

Took it down the garage, tank drained, new seal on the fuel sender, new seal and bolt on the drain, replaced diesel, everything tight - no leak, lovely, went home!

Next morning, up early, 7:00am, no leak - happy - back inside, breakfast, a little work, sun breaks out, check again at 11 o'clock - LEAKING???!!! what....

Back down the garage, brand new fuel tank fitted as the spot welds around the sender where corroded and couldn't tighten it anymore.

But by now id already made the link between the sun heating up the fuel and it expanding, thus pressurising it and the diesel and finding a way out of the tank.....

Now, i should mention here that my landy has always started very well, very quickly, only turning over for a few seconds before firing up.

The garage man said that drilling a hole in the cap to release pressure will fix it......hmmmm...... anyway, so be it, let's try, hole drilled

Now, i have no leaks, great, but i have the following 2 new issues

1) It now takes between 10 and 15 seconds of turning over before she starts

2) If ive been driving, then park up and idle for a few mins, then set off again, she struggles, fuel starvation, nearly cutting out, no revs, after a minute or so, everything ok again

Ive never had these issues before

So, in my tiny mind, id make the assumption that the fuel is draining back out of the fuel supply pipes back into the tank, which is prolonging the startup as it has to suck it up right from the tank (fuel lift connected to engine so it has to be turning over for this to work) - before, the little bit of pressure in the tank would prevent this from happening, keeping the fuel in the pipes...

So the question is, were the defender designers expecting the pressure there to help with this? thus why they sell the caps and sealed it all?

I would really like to get to the bottom of this, as drilling a hole in the cap is changing a design in my mind, and hiding a issue rather than solving it


Other possible areas where the problem could be i guess would be a malfunctioning fuel lift pump (lacking pressure) or maybe a fuel blockage in the filter or something from when the tank was changed.

Sorry about the long first post

Kindest regards

Billy

well as introductions go, that sucks, i got bored after line 3.

I would have thought that the cap was vented in such a way so as to relieve pressure (as youd imagine) then again i havent actually looked at it.

I suspect however that the tank is not airtight as when i changed my fuel tank for a new one (never again) the system didnt have to be bled and it started first time. And of course when youre filling up there is a big gaping hole allowing free airflow into the tank. Ive also ran it completely empty and again it started first time after filling it up, im guessing it would have tried to draw some fuel in and instead got air.

I suspect the problem is more to do with the actual fuel system itself than the tank and its surrounding components. Stupid question but have your tried changing the filter and bleeding the system to see if it makes any difference?

I'd be looking at the fuel pipes to see if I could see where the air is getting in from. As your fuel tank should be able to breathe anyway i'd also check to see that the breather isn't blocked.

pressure plays no part in holding fuel in system ,you need an air leak ,on leak off pipes ,banjo bolts ,pipes lift pump etc ,having no cap at all wouldnt effect starting ,

the_wolf said:

I would have thought that the cap was vented in such a way so as to relieve pressure (as youd imagine) then again i havent actually looked at it.

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If you take it apart you will see that there is a ball held in by a spring which can only be "sucked" open and not blown open

ratty said:

I'd be looking at the fuel pipes to see if I could see where the air is getting in from. As your fuel tank should be able to breathe anyway i'd also check to see that the breather isn't blocked.

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Yes, this i show my 200TDI worked, but on this 300 i see the breather is connected back into the top of the fuel filler pipe, unless there is another breather in the engine bay

And this is confirmed when you look at the workshop manuals

So the cap blocks that off too, but allows it to breath when filling up

Also, a well known way to know if the turbo diaphram is breaking, is that when you remove the fuel cap, it blows off in your hand - as the diaphram has leaked its pressure into the fuel tank - this again confirms that the fuel tank _can_ be pressurised

jamesmartin said:

pressure plays no part in holding fuel in system ,you need an air leak ,on leak off pipes ,banjo bolts ,pipes lift pump etc ,having no cap at all wouldnt effect starting ,

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yes having the cap off makes no difference to starting it now, as its had a hole drilled in it anyway - however, im tempted to block it again and let it pressurise again to really prove that once and for all

These two issues were never there before the tank was changed and the hole drilled, so its either a pipe problem in that area, a bit of crap thats trying to get through the pipes and blocking, or something to do with the internal pressure

I dont think i have an air leak inwards anywhere as i would expect that to show problems much more often then only starting and after a couple of minutes of idling - would you not? when shes running all seems fine, turbo kicking in, can get to 70mph...

it shouldnt have before either tank pressure has nothing to do with starting,tdi pumps are self bleeding so will run with some air been pulled in ,lift pump contains one way valves

jamesmartin said:

it shouldnt have before either tank pressure has nothing to do with starting,tdi pumps are self bleeding so will run with some air been pulled in ,lift pump contains one way valves

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yes of course, and after you have filled the tank and put the cap back on it will be de-pressurised and still starts - this i understand

but i have a list of things to try so i can find the fault, including checking filters etc, but im really after some concrete answers about the cap drilling bit here

So i believe that on these 300's; landrover intended the fuel tank to be sealed as it was on mine, and to be able to cope with fuel expanding pressure, which is why they developed the cap in the way they did and moved the breather like they did

I, as an engineer myself, cannot simply buy an answer of "drill a hole in the cap" without an understanding as to why - as this is in effect changing its design

Whether its the cause of my problem, or having a small part in my problem, or having nothing to do with it at all is what id really like to clear up - so as the subject line stats

A little pressure or no pressure in the tank?

Im in a mind to block the hole and do my fault finding like that, but maybe someone will pipe up with a late change modification or something to put me right

do any other 300TDI owners hear a tiny psst when they open the cap after a few days in the sun? has anyone else done the hole drilling in the cap?

the fuel tanks and caps are the same from early ones till td5 ,so not specially designed for 300 tdi ,i suspect you had leaky tank assembly and a good system wouldnt have leaked ,starting problem due to cooincedental air leak or caused by disturbing system ,lift pumps are fragile

billysdomain said:

Yes, this i show my 200TDI worked, but on this 300 i see the breather is connected back into the top of the fuel filler pipe, unless there is another breather in the engine bay

And this is confirmed when you look at the workshop manuals

So the cap blocks that off too, but allows it to breath when filling up

Also, a well known way to know if the turbo diaphram is breaking, is that when you remove the fuel cap, it blows off in your hand - as the diaphram has leaked its pressure into the fuel tank - this again confirms that the fuel tank _can_ be pressurised

Click to expand...

there is a world of difference to the amount or pressure created by a turbo and that created by the fuel itself.

If the fuel tank couldn't breathe you do realise that your fuel tank would colapse due to the vacuum created by the fuel being removed?

billysdomain said:

yes having the cap off makes no difference to starting it now, as its had a hole drilled in it anyway - however, im tempted to block it again and let it pressurise again to really prove that once and for all

These two issues were never there before the tank was changed and the hole drilled, so its either a pipe problem in that area, a bit of crap thats trying to get through the pipes and blocking, or something to do with the internal pressure

I dont think i have an air leak inwards anywhere as i would expect that to show problems much more often then only starting and after a couple of minutes of idling - would you not? when shes running all seems fine, turbo kicking in, can get to 70mph...

Click to expand...

Did you know that gas is smaller than liquid? This means that air can get in where liquid can't. Did you further know that vacuums suck inwards and not blowing outwards so it is far easier to suck air in without leaking fluid out. A diesel engine can and will run with air in it. You can see for yourself by fittig a transparent pipe if you want. Bubbles = air.

jamesmartin said:

it shouldnt have before either tank pressure has nothing to do with starting,tdi pumps are self bleeding so will run with some air been pulled in ,lift pump contains one way valves

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What he says ^^^^

billysdomain said:

If you take it apart you will see that there is a ball held in by a spring which can only be "sucked" open and not blown open

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If you take what apart?

billysdomain said:

yes of course, and after you have filled the tank and put the cap back on it will be de-pressurised and still starts - this i understand

but i have a list of things to try so i can find the fault, including checking filters etc, but im really after some concrete answers about the cap drilling bit here

So i believe that on these 300's; landrover intended the fuel tank to be sealed as it was on mine, and to be able to cope with fuel expanding pressure, which is why they developed the cap in the way they did and moved the breather like they did

I, as an engineer myself, cannot simply buy an answer of "drill a hole in the cap" without an understanding as to why - as this is in effect changing its design

Whether its the cause of my problem, or having a small part in my problem, or having nothing to do with it at all is what id really like to clear up - so as the subject line stats

A little pressure or no pressure in the tank?

Im in a mind to block the hole and do my fault finding like that, but maybe someone will pipe up with a late change modification or something to put me right

do any other 300TDI owners hear a tiny psst when they open the cap after a few days in the sun? has anyone else done the hole drilling in the cap?

Click to expand...

Do you have a picture of your fuel cap?

You do know there's more than one version, vented and non vented.

As an engineer surely you'd understand that as the fault only came on after you mucked about with the tank, there is a high likely hood that it was the mucking about with the tank that caused the issue.

ratty said:

there is a world of difference to the amount or pressure created by a turbo and that created by the fuel itself.

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Yes, as implied by using the word "blows"

ratty said:

If the fuel tank couldn't breathe you do realise that your fuel tank would colapse due to the vacuum created by the fuel being removed?

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I think if you read the posts correctly you will see that this has been covered, and is in fact all that the "vented" bit does of the vented fuel cap.

ratty said:

Did you know that gas is smaller than liquid? This means that air can get in where liquid can't. Did you further know that vacuums suck inwards and not blowing outwards so it is far easier to suck air in without leaking fluid out. A diesel engine can and will run with air in it. You can see for yourself by fittig a transparent pipe if you want. Bubbles = air.

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Yes this is all a known many thanks

ratty said:

If you take what apart?

Click to expand...

Fuel cap

ratty said:

Do you have a picture of your fuel cap?

You do know there's more than one version, vented and non vented.

Click to expand...

Yes, i have the black vented version, if i remember the unvented ones are _mostly_ grey - but dont quote me on that

ratty said:

As an engineer surely you'd understand that as the fault only came on after you mucked about with the tank, there is a high likely hood that it was the mucking about with the tank that caused the issue.

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I am asking a specific question regarding peoples experiences with fuel caps being drilled or not and the resulting pressure in the tank or not - im not here trying to poke for a specific answer to my fault - i want to understand the individuel parts of the system so i can get a higher understanding of the entire setup - in this, im seeking a greater understanding in how the system was in fact intended to be used, with pressure in it or not - its as simple as that, breath out any pressure to air (like drilling a hole in the cap) or keep it locked in

I am looking forward to hopefully finding a definitive answer to that one question

billysdomain said:

Yes, i have the black vented version, if i remember the unvented ones are _mostly_ grey - but dont quote me on that

I am asking a specific question regarding peoples experiences with fuel caps being drilled or not and the resulting pressure in the tank or not - im not here trying to poke for a specific answer to my fault - i want to understand the individuel parts of the system so i can get a higher understanding of the entire setup - in this, im seeking a greater understanding in how the system was in fact intended to be used, with pressure in it or not - its as simple as that, breath out any pressure to air (like drilling a hole in the cap) or keep it locked in

I am looking forward to hopefully finding a definitive answer to that one question

Click to expand...

If anything there should be a slight vacuum in the tank rather than a positive pressure.

All that drilling the fuel cap has done is to disguise the actual fault that is still there.

jamesmartin said:

the fuel tanks and caps are the same from early ones till td5 ,so not specially designed for 300 tdi ,i suspect you had leaky tank assembly and a good system wouldnt have leaked ,starting problem due to cooincedental air leak or caused by disturbing system ,lift pumps are fragile

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Yes there is a lot of sense here, your proberbly right and i agree, i dont think it was a waste to change the tank at all and now i have a good tank its showing a different fault, which was possibly there all along - or the changing of the tank itself could of upset something new as previously suggested and the tank was just a bum one which would have leaked anyhow (only had it 2 months)

But in that case i could buy a new vented fuel cap and replace my drilled one, allowing the expansion pressure to build again without it leaking out of the new tank, and continue to find my new fault

still not 100% sure about that

ratty said:

If anything there should be a slight vacuum in the tank rather than a positive pressure.

All that drilling the fuel cap has done is to disguise the actual fault that is still there.

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Yes - this was my exact thinking, but when told by the garage i had no reason to disagree at the time - vacuum which isnt quite strong enough to open the vent

But it was certainly the sun which started it to leak again, it warmed everything up and it created the positive pressure instead, as we monitored that