Fuel tank vacuum

I'm missing something somewhere: I can't stop a vacuum developing in the
tank. It has a lockable cap with breather valves, which I've stripped
down several times to check/clean. As far as I can see, and gently
testing with the end of a paper clip, the valves don't stick, and there
are unblocked air passages through the cap, yet it still has a stonking
vacuum when I come to re-fill it. I haven't blown then through yet, as a
mouthful of diesel doesn't appeal, but it doesn't *look* to be
necessary.

I doubt the vacuum is doing either the tank or the pump too much good,
but I'm not sure what to do about it. Am I missing a trick with the
filler cap? do the vents block/jam in some common but not-obvious way?

Baffled of Bristol.

PS: I've still got that new (petrol) tank - free to a good home - if
anyone wants to collect it.

--
simonm|at|muircom|dot|demon|.|c|oh|dot|u|kay
SIMON MUIR, UK INDEPENDENCE PARTY, BRISTOL www.ukip.org
EUROPEANS AGAINST THE EU www.members.aol.com/eurofaq
GT250A'76 R80/RT'86 110CSW TD'88 www.kc3ltd.co.uk/profile/eurofollie/

In message <[email protected]>, SpamTrapSeeSig
<[email protected]> writes
>I doubt the vacuum is doing either the tank or the pump too much good,
>but I'm not sure what to do about it. Am I missing a trick with the
>filler cap? do the vents block/jam in some common but not-obvious way?


What vehicle are you talking about?

Modern vehicles are s'posed to run with the fuel tanks under a slight
vacuum. When you have a crash the fuel is less likely to spray out on
impact.

How much of a vacuum do you have?

--
Marc Draper

In article <[email protected]>, Marc Draper
<[email protected]> writes
>In message <[email protected]>,
>SpamTrapSeeSig <[email protected]> writes
>>I doubt the vacuum is doing either the tank or the pump too much good,
>>but I'm not sure what to do about it. Am I missing a trick with the
>>filler cap? do the vents block/jam in some common but not-obvious way?
>
>
>What vehicle are you talking about?

110 2.5TD circa 1988.
>
>Modern vehicles are s'posed to run with the fuel tanks under a slight
>vacuum. When you have a crash the fuel is less likely to spray out on
>impact.
>
>How much of a vacuum do you have?

Enough to make the tank ping back into shape when I take the cap off.


Regards,

Simonm.

--
simonm|at|muircom|dot|demon|.|c|oh|dot|u|kay
SIMON MUIR, UK INDEPENDENCE PARTY, BRISTOL www.ukip.org
EUROPEANS AGAINST THE EU www.members.aol.com/eurofaq
GT250A'76 R80/RT'86 110CSW TD'88 www.kc3ltd.co.uk/profile/eurofollie/

In message <[email protected]>, SpamTrapSeeSig
<[email protected]> writes
>110 2.5TD circa 1988.
>>
>>Modern vehicles are s'posed to run with the fuel tanks under a slight
>>vacuum. When you have a crash the fuel is less likely to spray out on
>>impact.
>>
>>How much of a vacuum do you have?
>
>Enough to make the tank ping back into shape when I take the cap off.


Are you sure it is a vacuum? And not pressure building up inside the
tank ?

It is common on the 2.5TD for the boost metering diaphragm on the
injection pump to split, this results in the fuel tank getting
pressurised by the turbo. In extreme cases the engine will rev up of
it's own accord if left idling for some time.

Good luck
--
Marc Draper

In article <[email protected]>, Marc Draper
<[email protected]> writes
>In message <[email protected]>,
>SpamTrapSeeSig <[email protected]> writes
>>110 2.5TD circa 1988.
>>>
>>>Modern vehicles are s'posed to run with the fuel tanks under a slight
>>>vacuum. When you have a crash the fuel is less likely to spray out on
>>>impact.
>>>
>>>How much of a vacuum do you have?
>>
>>Enough to make the tank ping back into shape when I take the cap off.
>
>
>Are you sure it is a vacuum? And not pressure building up inside the
>tank ?
>
>It is common on the 2.5TD for the boost metering diaphragm on the
>injection pump to split, this results in the fuel tank getting
>pressurised by the turbo. In extreme cases the engine will rev up of
>it's own accord if left idling for some time.

Thanks for the warning.

I'm almost certain it's a vacuum, since there's no smell when you loosen
the filler cap, and no detectable outrush of air. I will check, but I've
not seen an increase in revs after idling.

It still mystifies me though: the valves inside the filler cap for
pressure relief are clearly designed to work in both directions - relief
for both over- and under-pressure. Mine just don't seem to manage either


Does anyone know if this is this a common problem with the standard,
lockable filler caps?

Regards,

Simonm.

--
simonm|at|muircom|dot|demon|.|c|oh|dot|u|kay
SIMON MUIR, UK INDEPENDENCE PARTY, BRISTOL www.ukip.org
EUROPEANS AGAINST THE EU www.members.aol.com/eurofaq
GT250A'76 R80/RT'86 110CSW TD'88 www.kc3ltd.co.uk/profile/eurofollie/