I posted this same thread on jeepforum.. but lets see if you guys can help!
So lets see if you guys can figure out this one!

OK.. So I just swapped out the engine in my 00' XJ with one from an 01'..
When I start the darn thing it wants to act like the gas peddle is to the floor. it just revs out of control.. When this happens the throttle body is CLOSED. It acts like it gets way too much fuel and not enough air.
Since I have 2 engines I swapped out the entire throttle body along with all the sensors and surprise surprise it still does the same thing...
What am I missing??? is there a difference between the 00' and the 01' I don't know about??
There has got to be SOMEONE who has run into this problem before!!
even off the wall input is welcomed! anything that might point me in the right direction!!

 

Sounds like your looking at an air leak after the throttle body.

1. Check that the injectors are fully seated and that the orings are OK.
2. Check that the intake manifold is properly tightened.
3. Check all fittings on the manifold to make sure they are tight as well.

 

check you tps as well. just b/c its a 00 or 01 motor that does not main a think i have batterys that have laster years then i have had batterys that lasted a few weeks. that where bot bran new.

my 90 has a prob with killing the tps

 

I've tried 2 different TPS sensors to no avail.. that was my very first guess..
The guys at JF are saying it's some sort of vacume leak.. the intake isn't properly seated to the head etc..

any other possibilities? I'm just looking for as many angles as possible so I can try them all..

 

Leak at the head/intake.
FWIW, you might have the wrong int/ext. gasket.

 

Yes it sounds like an air leak in the intake manifold. Probably a dumb question but you did use a new intake/exhaust gasket when you installed the intake and exhaust right?

 

Strange. If it were a simple vacuum leak, you'd think that it would just stumble and stall out from being too lean. Something is telling it to add fuel as well.

When I swapped out the bad IAC on my '97 last week, I installed the new one and plugged it in. Upon restart, the motor revved to about 3500 rpm for about 25 seconds until the little servo motor had time to reset the position of the IAC plunger properly. It has idled perfectly since.