There are few possibilities , that needs to be checked.If there is steady check engine light coming up on dash, then get the car scanned.At autozone service center its absolutely free to get car scanned,.Yes , they do car scanning for free.Once its scanned , it will give the error codes.So as per error codes , it will point out, what is causing the high idle problem.-----------
BUT in case if there is no steady check engine light coming up on dash, then then in that case the IAC valve ( idle air control valve) , EGR valve, vacuum leaks, low fuel pressure, dirty throttle body or MAF sensor( mass air flow) and TPS ( throttle position sensor).-------
This are the possibilities that can cause high idle problem.-----------
IN most of the cases its faulty MAF sensor, that causes this problem.So start with MAF sensorr cleaning and see if that makes any difference.Also test the MAF sensor and see if its getting voltage to its terminal.---------- Disconnect MA sensor and see if idle gets better without MAF sensor.IF it gets better, then replace the MAF sensor.--------
The sensor is located between the throttle body and the airbox.--------
But if MAF sensor replace or checked ok, then in that case.------------
Other mentioned possibilities needs to be checked, click this link below for more details:--------
http://technoanswers.blogspot.com/2011/09/1999-chevrolet-monte-carlo-idles-high.html
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This will help.Thanks.Helpmech.