idle could be a few things .
start with trying to remove the oil cap with it ticking over. see what it does to the idle.
should be no more difficult than removing it when the engines off. and it should only upset the idle slightly.
but if the cap is held down by vacuum within the engine crankcase. and on removing it if the car near stalls with a big rush of air thru the open oil cap..... then you need a new pcv valve unit thats attached to the cam cover.
as thats where the engine is finding the unmetered air to upset the idle.