[Mk2] [Turbo] Causes of a high idle?

[Harold]

After having replaced the rocker cover gasket the idle has been slightly higher than normal. From cold it will idle at 1500 rpm and then drop down to around 900, previously it was perfect, idling at 1250rpm when cold and dropping to 750.

I did give the coolant a bleed as I disconnected some coolant lines, a tiny bit of air came out and it has made no difference to the idle, the car drives fine with no overheating at all. There are no ecu error codes.

What could the causes be?

Edit:
Have double checked that everything is tight and screwed in properly, no loose vac lines, The throttle body gasket is in good condition also. I did try to remove the idle control valve so I could clean it but the Phillips screws holding it in were seized to bu88ery and just wanted to round off.

However it can't be due to a dirty icv as it was fine beforehand, could it be due to some air still in the system?

[RobPhoboS]

When I had a high idle issue (for 1.5 years!) it turned out to be the garage which put the wrong thermostat back in my car, one which runs a different temp to standard.
Could be worth checking that out.

[dantheman]

throttle cable pretention? by that i mean when you put the throttle cable back did you tighted up the 2 bolts so that you always have a small amount of throttle cable tension?