Replaced AOS, but car is idling rough

[mkc2bad]

Hello Everyone!

My car was making the belt squeaky sound very loudly, removing the oil filler cap elimited the noise. I therefore thought the AOS was failing (in part it may have been) I replaced the AOS after following the excellent guide as linked on from this forum - However the squeaking noise is still just about audible (much much fainter than before) The car is also idling very roughly anytime you come off the throttle. It tends to sort itself out after 5 seconds or so but I can tell something isn't quite right - feels slightly more brash and lumpy, rather than the silky smooth feeling it had before. It also caused the check engine light to come on - I've since reset the battery as I took the AOS out and re connected it all fearing I had not connected something correctly. I'm 100% sure everything is now connected properly. I was given a new rubber hose bit with the AOS (is this the bellows??) that connects right at the bottom of the AOS. I replaced the spring type clamp with a jubilee (screw) type clamp.

Any ideas as to where the problem maybe? Would it be worth replacing the plastic Y tube that connects the AOS to the throttle body? At a guess I'm assuming there is a vaccum leak somewhere as when I take the oil filler cap off the car idles much more smoothly.

Thanks

Mike