How did you go about checking for leaks? Visually or with starter spray?
If former, buy some starter spray and spray all around the hoses and listen for increase idle. Some cracks are so small in those rubber hoses that you can't see them.
I had an idle surge problem that was cured by cleaning the idle valve with laquer thinner (fill it up and shake it well) and then lubricated with WD-40.
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi