Just replaced the head gasket on a 2000 Jeep Cherokee XJ 4.0L. After reassembly the motor wants to idle to what seems to be the rev limiter ( I shut it down at around 3-4k rpm every time). The throttle body also isn't opening so I am assuming there is a huge vacuum leak somewhere, my best guess is the exhaust/intake manifold. Figured I'd post to see if anyone else has had this problem before I start tearing it apart again. Also I cleaned the intake with degreaser and water before reinstalling, so maybe I could have got water in an electrical connection on the intake? Any help would be appreciated ,Thanks.

 

The intake may not be properly "seated" and is not flat on the head even though you think it is. Intake must be properly seated on the dowel alignment pins.

That is a very common cause of the extremely high rev you are experiencing after head work. You essentially have a huge vacuum leak.

 

I had a similar issue and had not torqued the bolts down far enough, Worth checking since it is an easy fix/something to eliminate possibility wise. While it's idling, spray carb cleaner or something along the gasket. This will essentially fill the gasket if there is any "room". If things are not tight enough, this will drop the idle for a few seconds until it drys up again.

For reference, I had used torque specks I got offline and ended up going well past it to drop the idle.

 

You definitively want to check your intake manifold and make 100% sure it is aligned on the dowels. I had the same problem and thought it was on 3 different times. It will sir on top of the dowels and seem like it is on. That little bit of misalignment will cause an airleak and the super high idle.

 

The intake manifold is hanging up on the locating dowels.

 

Update:
Thanks to all who replied, it ended up being that the dowels were not aligned properly, creating the vacuum leak. Thanks to all who replied.

 

Good to hear its sorted.

 

Cruiser, You need a cape and spandex! You've had the right scoop EVERY time I've listened to you.