From what I've read elsewhere, the cheapest thing to be wrong would be a
leak in an air hose (after the air flow has been metered). If you can't find
it by manipulating the hoses, apparently spraying carb cleaner in the
vicinity will change the idle when it's sprayed near the leak - it could be
a gasket/O-ring rather than a hose.
I had an Astra with the same problem, and it did have a split hose, but I'm
not familiar with the arrangement of hoses in your car.
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.