Idle Problem 1995 Grand Am

I own a 1995 Grand Am 3.1L V6 with 161,000 miles on it. It has been running great with no problems until today. When I let the car sit in Park or Neutral, it idles higher than it should and eventually the service engine soon light comes on. Once the car is in gear the idling is fine. The only changes that have been done to the car is a higher flow muffler (flowmaster 50 series) 8 monthes ago. Can anyone tell me what this might be? Thanks.

bad pcv valve, air leak in one of the hoses
or dirty idle motor. ?

Does sound like a vacum leak, and or your IAC is not functioniing properly.

You can remove it and see it cleaning it helps.
Just dont push/pull or twist the pointed tip(the pintal) this will mess up the calibration making your idle worse.

Make sure the port it sets in is clean too.
Alot of times carbon builds up and will cause idle to do all sorts of weird things.