Well I recently bought a used 1999 GT Vert, 29,000 miles with a 5speed manual. It runs fine except at idle. When I first turn on the car in the morning it usually drops in RPM's and stalls. When I start it a 2nd time, the car stays on but the RPM's surge from about 500 to 1500 at idle and continues to do this for the duration of the trip. Does anyone have any idea what is causing this? I recently installed the Predator performance tune, but it was doing this before I installed it, however, it did not seem as bad and it did not stall. Any help would be greatly appreciated. Thanks.

 

IAC or a bad TPS.Spray carb cleaner into the hose going into the IAC valve to clean this.The TPS has a carbon film that can wear through.When the pickup goes over the bad spot the signal is lost from the TPS and the engine computer doesn't know how far open the throttle is.Replacement is the only fix for a bad TPS.

 

Thanks for the info. But I have a few questions: What is the IAC and where is it located? and what is a TPS and how much does it cost to replace?

 

The IAC is the idle air controler. The TPS is the throttle positioning sensor. Both are on the on the front of the plenum, the factory plenum is what the mustang symbol is on under the hood. The iac will be located on front/right side, looks like it has a little electric motor attached. The TPS is just a plastic sensor that is on the front/left.

both are about 40

 

This thread will take care of ya.

https://mustangforums.com/m_587901/tm.htm

 

what should the IDLE be running at?

 

Mine is around 700rpm. I just replaced my IAC becaues my car was stalling.