Ok, so I think I posted something about this here before but I think it might have been at a different forum. Anyways. I have an 02 Mustang Convertible Automatic (yea, i know). I have installed an air intake where the filter sits in the fender. It doesn't go down into the fender but it does draw air in from the fender.
When I start the car, the car revs up as normal. The needle starts going down but then hovers from just under 1000rpms to 1200rpms. This was happening before but it once went all the way up to 2000rpm. I took out the filter, cleaned it, and reinstalled the piping and everything. During all this time the battery was disconnected and I believe the computer reset as well. I started the car and it ran fine. But today it started acting up again when I started the car hovering around 1000rpm. It used to idle at around 600rpm. I've had the intake in there for almost 2 years now and this problem started appearing around 8 months ago. Sometimes it dissapears for a while and then comes back. Any suggestions of what the problem could be? My repair manual suggests intake leaks and filter cleaning. Thanks for any help.

 

Also spraying your MAF sensor wire with electrical cleaner and replace your fuel filter.