E39 540 strange idle depending on fuel level

[Greg130]

Hi Guys,

1998 pre vanos 540

Runs and drives lovely but has a strange idle issue (checked for vacuum leaks, has had full set of new intake gaskets plus a heaps of other stuff replaced)

Anyway when warmed up and with a full tank of fuel she idles at @ 550rpm, engine sounds spot on, really couldn't fault it.

As soon as the fuel level drops below half a tank she idles at @ 750rpm, still sounds ok and drives fine but it annoys me.

So any ideas why it should idle @200rpm higher with half a tank of fuel?

Got me stumped. (no surging or spluttering ect and car drives just fine)

[Greg130]

Any ideas, is this normal?

[topaz540i]

Thats the rpm i idle at when a/c is on