Has anyone heard about leaving cap abit loose to let air in tank?

 

I haven't, but what would this achieve?

 

To stop a vacum in the tank as the petrol gets used up. Not sure with standard pumps maybe just the uprated ones

 

That's interesting.

When i undo my cap I can hear fast moving air, but is that fresh air being sucked into the tank due to a vacuum, or vapours being pushed out?
I think it's vapours being pushed out?

 

the vacume is suction surley? you imagine sucking a carton it squeezes closed.

Having the cap open or loose and driving at speeds can only encurrage evaporation of very expensive fuel.

 

Or letting moisture in?

 

It's a puzzling idea in my head now lol

 

Funny that this shud come up today.

Some months ago i had my fuel pump out to check it (prob turned out to be immobiliser.....!) ; Following this, I had lost my 'suck'! (I just ignored it), then i noticed that my fuel comsumption was suffering (fuel evaporating ??).

Yesterday I had the pump out, and applied some sealant to the seal - I now have my suck back and will be doing a check on fuel comsumption to see if it has improved...............