So just drive about 15 miles to school and it's about 58f out rn first chilly day this year and I was sitting in the parking lot and all of a sudden the jeep starts revving out to about 1800 rpm it did that for about 30 seconds then I turned it off, I waited a little while then turned it back on and it turns over and starts at about 2k rpm then I heard a loud pop and it has gone back to its normal at about 900rpm. What could have caused the jump. 1996 xj country 240k on engine...

 

One thing that can cause high idle is vacuum leak in manifold.

 

hmm good to know but this is the first time it's done it... But there is a first for everything. Now I did here a loud pop and it sound like somthing fell out of the engine bay, I checked and found nothing. -edit- also what's the best way to check for a leak

 

found multiple cracks in the manifold and it was dented really badly at the end... Well new exhaust system up next

 

I think he meant your intake manifold. but sounds like you may have found the problem anyway. as long as it doesnt happen again i wouldnt worry about it.

 

It actually just did it. Every time I'm sitting in park for more than 5 minutes it does it

 

I was referring to intake manifold, however if exhaust manifold is cracked than air will be let in (and exhaust out). This causes the O2 sensor to read lean and increase fuel flow unnecessarily.
If the crush/dent you are referring to is in the downpipe of exhaust, that it is supposed to be there. Something about bringing the catalytic converter up to operating temp faster.
A thorough visual inspection with a good light is always a good idea.

 

It dosnt look that bad on camera but it's a nice dent

 

The pic you posted looks to be freshly scratched or impacted. Did the driveshaft knuckle contact exhaust?
Attached is a picture of the factory dent I was referring to. Located just beyond exhaust header, in top portion of downtube.

 

Well I found out my 13 year old brother took a joy ride the night before.... That might explain that, someone's going to get a a$$ whooping when I get home

 

Uh-oh, may want to inspect front passenger side of oil pan and transmission lines. When suspension bottoms out the upper control arm can make contact with oil pan flange/bolts and pinch the transmission lines that run alongside. There is a reference picture in first post of linked thread. https://www.cherokeeforum.com/f2/che...-lines-221708/