I have a 97 4x4 with the 3.9 in it. When I start it up it idles fine around the 600 rpm range in park. When I'm in drive and come to a stop it idles fine around the 600 rpm range. But after driving for a little bit then placing it into park again it idles at 1100 rpms. So I shift it into drive and the idle drops to 600, neutral its back at 1100 and park at 1100. What could this be?

 

idk why it does it but most vehicles with an automatic will have that same sort of thing happen to it.

My dad's dak does it, his volvo, mom's van, buddy's trucks, just about everything with an auto has it happen.

 

Check your idle air control solenoid. im not really sure if its a solenoid or a motor, but this is what makes it idle higher in the morning when its colder, and sometimes when you have your a/c on or other thing demanding the engine to work harder.

 

vacuum line that is leaking could very well do this

 

Yep sure is, I popped the hood on it the day after I posted this and checked all my vacuum lines going to the intake. Turns out the vacuum line from the intake to the PCV valve was cracked. Replaced it and it's back to normal. Thanks for the help.