Low Idle

Ok, here's the skinny: My 88 XJ would sometimes idle at like 300 rpm, and sometimes key up to like 3500 rpm with little or no notice. I replaced the TPS which made it run a heck of a lot smoother and fixed my high idle problem, but now no matter where i put the idle screw, it remains at like 300 rpm. Any ideas?

That TPS is adjustable but I'm not sure if it will change the idle speed.

It will not change the idle speed...

I wouldn't recomend using the idle screwin anything but dire circumstances.

What about your voltage? When my alternater wasn't putting out enough voltage at idle my idle would drop.

You coming up ti Big Bear? If so we can stare at it and scratch our heads.

I am indeed! I would really appreciate that, this is my first fuel injected vehicle, I'm used to Holley or Carter or even Quadra-Crud carbeurators, which in my opinion is more easily diagnosed. I'll be there from Friday afternoon through Sunday. The good part is it DOES idle now instead of stalling, but man, I'd really like it around 600 RPM or so.

Your idle speed is controlled by the ECM. The idle adjustment is intended to set what is called "minimum air", not to set the desired idle speed; you should not fool with "minimum air" to compensate for a problem in the idle circuit. The AIC (air idle control) motor, located on the throttle body, responds to ECM output. The AIC has a plunger that controls the amount of air bypassing the throttle plate. Too little air, low idle, too much air, high idle.
Your problem seems to be too little air. This indicates a bad AIC (quite possible), a plugged bypass passage in the throttle body (unlikely),a bad connector on the AIC (remove and clean contacts), or a bad ECM (not likely).
The AIC has two sets of windings; one set to retract the plunger (more air bypass, higher idle), and one set extend the plunger (less air by pass, lower idle). The ECM send pulses to the AIC to effect retract and extend. An inexpensive LED voltage tester will tell you if the ECM is talking to the AIC--back probe the connector, one set of windings at a time, while the engine is idling; you should see the light blinking rapidly. This won't guarantee the ECM is good, but I'd bet it is, as is the wiring to the ECM if you see the blinking.

I highly doubt the ECM is bad. I've heard of some two bad Renix computers in all my years around XJ's.

Also in my experiance with Renix Jeeps bad IAC's tend to produce an unsteady idle, I haven't had one completely fail out. Also they're generally just gummed up and easily repairable in my experiance

(That's one of the things we'll be checking up on the mountain, another possiblity is the C101 if he's got an early '88)

Like I said "not likely" for the ECM. My money's on a failed AIC, whether it' totally caput or "gummed up". But personally, I'd clean the connector first, test the ECM outputs next, the clean the pintal on the AIC next.

1 shot of wd40 down the shaft of the IAC might also help too, after cleaning the head.

Sounds like I'm talking about a penis doesn't it?

In my 88 the idle goes down as the temperature goes up. No matter what I do it seems to eventually reach the 400 or so mark. It just takes longer now.
What happened was I fubared my TPS at the quarter wash (engine wash), Idle went way up. I put in a new TPS and adjusted it by the book to the tranny side of the TPS. Idle was OK but still a little high. I finally adjusted it so the ECM side of the TPS read. .75 volts and the tranny side of the TPS reads 3.8 volts. In effect I split the difference.
Idles right around 600-650 now, except when it gets up around 220 F or so, then idles back down to the 400 mark. Another thing I noticed was that my mileage increased about 6%.
Darned Renixs are temperamental thats for sure.
I have two different base voltages going to my TPS. One from the ECU and the other from the TCU. Kind of makes it impossible to get both sides of the TPS adjusted to spec.

Thanks for the help guys! Some good stuff there .