Puzzling Idle (yes--I've searched)

OK, first things first, YES, I have searched the forum and can't find anything that matches this.

The specs: '89 Cherokee Laredo, 4.0L, NP231, AW4, stock everything

The problem: high idle (1500-2000 rpm out of gear) when warm (500-750 when cold). If I start the jeep when cold and let warm up, the idle climbs up to around 1500-1750 and stays there. This takes several minutes. If I then kill it, wait several minutes, and restart it, the idle starts at about 500 rpm and eventually climbs to 1000 rpm (fine by me). If I then drive it, the idle will climb to 1500-2000 rpm and stay there until I repeat the sequence.

What I've checked:

• TPS--not replaced, but smooth and even across range and properly adjusted
• MAT--resistance is in spec as the engine warms up
• MAP--source is within spec, output is low once engine is warm (per Mitchell OnDemand specs--spec is 1.5V to 2.1V, reading is 1.35V)
• IAS (IAC, ISS, whatever--the Idle Air Servo)--replaced--not sure how to verify operation
• EGR control solenoid--appears to be working correctly, need a guage to verify
• Intake Manifold bolts--tight
• Blocking off IAS input hole--no change in idle

What I know I need to check:

• O2 sensor (a little hard with no jack in a cold garage)
• CTS (same story--going to try getting readings at the ECU)
• Fuel pressure (specifically the regulator--need to rent/borrow guage)

Jeep starts and runs fine (just high idle). I just want my fuel economy and braking to be better (high idle causes surges in braking). I can give exact values for most of the sensors if people want/need them to help me figure this out.

Thanks,
Chris

You sound just like I was last year, but my intake manifold bolts were loose. I am going to say that the intake manifold gasket needs to be changed. I know of only one way to check this.
With engine running(preferable low idle), spray something flammable(PB blaster, carb/choke cleaner) where the intake manifold meets the head. If the idle jumps then there is a leak. Before somebody says that is stupid, yes be careful, the exhaust is right there so there is a possiblity of a flame up. Just be careful and you will be fine.
Maybe there is a way to test the vaccum?
If it is not that I do not know what to tell you.
Good luck
Katarn

I'd first plug off all the vacuum lines on the intake, just for a test. Check the throttle plate and throttle cable for sticking.
When you replaced the IAC did you clean out the seat area? My IAC works a lot better since I cleaned the connector with a small nylon brush. And my hot starts have dropped to almost nothing, since I cleaned all the connectors on the lower left of the motor. I'm guesing it was the engine temp. connector that was causing problems.

Get it warm and idleing at 1000 or so, then unplug the IAC. If it still goes higher then
you have a vac leak, or your throttle plate isn't fully closing for some reason.