High RPM on 2000 Elantra

I have 2000 Elantra. After sitting approximately three hours engine will always rev up to 3000 RPM and climbing. I always press lightly gas pedal, increasing RPM even higher, forcing the engine computer to
"reconsider" idling RPM. Normally, RPM is around 1500 and dropping to 500 when warm. It appears to me as a software bug. Is there any update for this? Mechanics could not find anything wrong with the engine. (2L engine and manual, no air or nothing). And it is hard to repeat as it needs exact situation. If conditions are slightly different, everything is OK.
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This sounds like perhaps a vacuum leak or throttle position sensor issue. In many cases, the throttle position sensor will leave a trouble code but not set the check engine lamp.
The most frequent vacuum leak is in the PCV hose.
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hyundaitech wrote:

happens only under very special condition. Perhaps occur only once every three months. And problem is there since the new.
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