Vacuum leak most times.
The hose that makes the car breath right?
enginecreator is correct. It's one of the main cause for high idling. It's not the big hose that connect from the filter to the engine. it has to be one of the smaller hoses connecting to the intake (or anywhere after the Mas Air Flow sensor). Good luck:-)
Yep a brake booster can be cause of the leak too. I suggest open hood crank engine and listen for a hiss, try to find where its hissing from. If its a hose, its a cheap part to replace.
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