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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.

Reply to
SpamTrapSeeSig
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In message , SpamTrapSeeSig writes

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?

Reply to
Marc Draper

In article , Marc Draper writes

110 2.5TD circa 1988.

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

Regards,

Simonm.

Reply to
SpamTrapSeeSig

In message , SpamTrapSeeSig writes

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

Reply to
Marc Draper

In article , Marc Draper writes

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.

Reply to
SpamTrapSeeSig

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