I have a 2000 MK1 1.6 Zetec, and something strange is happening. After I refuel my rev's then sit around 3k and takes about 5 seconds before it drops to about 1.5k and sits there, its also pinging as well when pulling off in first gear during this time. After a few days it returns to normal.

Something to note is that every time I undo the fuel cap there is a rush of air, not sure if its in or out though.

I thought it could be a fuel tank breather hose but then i thought it would be better when the cap had been removed as the pressure would be normalised and get worse over time rather than the other way around?

Has anyone come across this or have any ideas what could be causing it?

Thanks!