Pontiac Repair: 1998 monte carlo, c lines, engine computer

Question
Todd, I've got a 1998 Monte Carlo with 3.1 engine. Sometime when I start it up the engine revs high betwenn 1500 - 2000 rpm. Normally it starts up running about 1000 rpm. It seems like it happens more frequently after sitting all night but has been happening more and more frequently like after stopping at the store & coming out around 1/2 hr later. It will not kick down on its own but will usually drop down to normal if I turn it off then restart but this is not always working any more and it seems to be getting worse.Any ideas? thanks. Dave.

Answer
Hi David:

those engines would do odd things like that and there were a multitude of reasons for it. firts try to clean the throttle body and Idle control motordi connect the battery and let it sit. for a few minutes. reconnect the battery. turn the key to run do not start it. turn the key off wait 1 minute then turn it to run again and then off for a minute. then start the car. thatwill reset the idle control motor. Other things were the TP sensors would go bad from time to time do to the heat soak. another issue was the A/C  pressure senor on the HBi side A?C lne would go bad and it will effect the TP sensor. try unplugging that sensor and drive the car a few days and see if that fixes it. The sensor has three wires on it and it's on the smaller of the 2 A?C lines on the driver side of the car by the air cleaner assembly. Last idea is a bad Engine computer but that is bway down the list of reasons. Good luck :)