Odd high idle trouble... help please!

I have a 1998 XJ, 4.0, AW4 with 108K miles that is having an odd high idle problem. CEL codes thrown are 122,123,1694. I have searched previous posts and have changed my IAC valve (cleaned port too) and TPS. I have checked for vacuum leaks with a non-ignited propane torch. My TPS signal voltage is reading 1.04 (a bit high?) - 4.0 throught the range, without drop outs. The source voltage is 5.15 (again, a bit high?). My TPS ground is normal at 0 OHMS. The IAC and TPS are new OEM Mopar parts. I am going to start checking grounds, connections and intake/exhaust manifold bolts next. The ground strap from the rear of the block to firewall "appears" good, but it will be checked momentarily.

My symptoms are: High/bouncing RPMs after warming up to normal operating temperature. RPMs at idle in park or neutral are 1,000 - 2,000. Sometimes they stick at around 2,000 and sometimes they drop all the way down to about 450 for a split and immediately bounce up to 1,500 - 2,000. Sometimes the RPMs will just hang out at 2,000 at idle indefinitely and at other times they will bounce up and down from 450 (almost stalls) to 2,000 in fairly rapid cycles.

While driving I can feel the same cylcles occuring. I can feel the drop off, like my fuel or ignition has been turned off, but if I "mash" the throttle it immediately responds very well. They Jeep seems to shift well and at the proper times and has no trouble going 80MPH or more. If I keep the pedal down and the vehicle is accelarating the problem does not seem to present itself, however if I try to maintain a constant speed I can feel the "drop off" and "bounce back". It's not a "spit and sputter" type issue at all. It's more like and "on.... almost off.... then over compensate for almost stalling" type issue, if that makes any sense.

Any help would be greatly appreciated guys!

Are you still setting fresh codes for the TPS after you've replaced it? If so, I would concentrate on the connector to the TPS (flush with electrical contact cleaner, inspect contacts, dab of dielectric grease and reattach) and also the TPS wiring. The OBDII system isn't perfect but it does a pretty good job of detecting a fault in a subsystem/circuit, and remember that the circuit is more than the sensor itself. Connections/wiring often overlooked.

What is the code 1694 related to? Not in my manual.

Question: Do you see these same symptoms when the engine is cold? What I'm getting at is the difference between open loop and closed loop. Open loop (cold) uses default values for fuel air mixture. Closed loop uses input data from the oxygen sensor. If you see symptoms ONLY when engine has warmed a bit, I'd concentrate on all things related to closed loop.

Getting a scan tool on it and looking at values real time may be of value.

i had similar thing goin on just not as bad, finally took it to my buddies shop and he threw the computer on it, put in drive with E brake on started setting it to proper idle and did an air bleed on the TB bent my linkage almost none and told me to peel-out, so do your checks and if nothing changes have a computer plugged into it! i worked on it for a day and a half, it took him 10 min...

I changed out my TCM to deal with the 1694 code after some research indicated a bad TCM. My 1694 code is now gone. Swapped out the PCM with one that I have in a 1999 XJ, 4.0, AW4. This made a world of difference. It's running very well now and Code 122 (TPS low volatage) is now gone, but I still have code 123 (TPS high voltage). I confirmed that the brand new, OEM TPS is reading 1.04 at minimum throttle opening to 4.0 at WOT on the signal wire. This is indeed high on the minumum side as the code indicates, but I can't figure out why. It's also low on the WOT side. The new TPS has identical reading as the old one and it doesn't matter which PCM is being used. Both PCMs have a source voltage of 5.15 volts too, which seems coincidental to me unless source voltage is not supplied by the PCM, but by something esle instead. Any suggestions?

Maybe there is a slight physical problem with my throttle body itself. I will check to see if my plate is actually closing all of the way. My TPS would be reading accurately if my plate was slightly open at idle. Maybe it's not opening all the way at WOT either. That would sure explain a lot, although it sure seemed like it was closed all the way from what I can recall.

I went through high idle problems with my 90 laredo. PITA. Replace all vacuum lines, replaced both the IAC and TPS but all of this seemed to help only a bit. I then added a throttle return spring from summit racing. That helped out a lot more but I still had some high idle at times and if i remember correctly it was usually when it was colder out.

I finally tracked it down with the propane trick. The manifold bolt closest to the firewall on mine was broken off inside after about two threads. A new gasket, a 90degree drill and a helicoil fixed mine. I know you did it but you might try again...it seems to me the idle only slightly crept up with the propane treatment to the area.

HTH

Darren.