Low idel runs fine

89 4.0 auto. The rpms hover around 250-300 which is kinda low. I tried adjusting the tps it either stays where its at or up to 2k there is no in between. If I leave it low it runs and drives fine. I do have a crack in the exhaust manifold. Is this a TPS issue or do I have other problems?

The IAC stepper motor should be giving you the correct idle, and that would be 725-750 rpm, in drive (friend sitting in driver's seat with foot on brake, parking brake on), engine fully warmed up.

The IAC may not be functioning correctly, either bad or just dirty. Do a search for IAC.

Also, there is a small allen screw (3/32" on my 90) that allows you to set a base idle for the XJ.

If yours has been adjusted in the past, perhaps by someone trying to compensate for an intake leak, then it will have to be reset.

First, remove the rubber intake hose. Good time to spray some TB cleaner around the throttle plate area. Ok, now take a look at the throttle plate. It should be almost fully closed, as the IAC should be able to give you an accurate idle, but adjusting it slightly is Ok--it should just start opening the throttle plate.

Don't forget to check/readjust the TPS.

Ok I pulled the IAC and cleaned it up, wasn't real bad but was pretty dirty. Reinstalled didn't really make a difference. Tweaked the tps a little and it got a bit better. I put a turn and a half on that adjuster and still sitting at 500 rpm. I need to get a multimeter and set the tps properly. I have that exhaust leak also not sure if it has anything to do with it but Im sure its not helping anything. I have a replacement manifold ready to go next weekend I'll change that out and then set everything. Fingers crossed

Homebrew.... I know this is an old post but any updates on your low idle??? I'm dealing with the same problem on my 91. Idles about 475-500 rpm warm.