Does anyone have any idea what would be causing this for my buddy's 1998 Cherokee?

Check it out:

http://www.youtube.com/watch?v=M17xI...ature=youtu.be

No one was in the Jeep touching the gas. It was just doing this on its own.

 

My guess would be it's saving fuel since it only runs half the time. Has it ever had any type of maintenance?

 

We thought maybe it was one of the Oxygen sensors, so we replaced that today.

This was really weird because it was idling and then started doing this. Literally the RPMs were going from 800 to 300 and back up then back down as you see in the video. If you punched the gas, everything ran fine, but it would start doing this again as soon as you let off.

I put it in Neutral and the Jeep completely stalled and died. It fired back up and the revving was gone.

It does this all the time intermittently. So we are having trouble diagnosing it.

 

Test the TPS sensor.

 

Do a thorough cleaning of the throttle body and pay close attention to the IAC, he might need a new one.

 

yes. check the TPS, IAC, and check all the air ducts/ everything before the intake manifold. Also check all the vacuum lines, because an engine will move/ twist in different ways. also, whats the temp of the truck when driving/ does it have a normal operating temp?

 

Runs below 210. We will take a look at the IAC today. Thanks for the suggestions.