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I hate to bring this up again, but I need help. I have a 1990 2.5 TBI wrangler that is killing me! I say I've been throwing parts at it, but given it's age everything I've done was either bad or well on its way. Now that almost everything has been replaced including the IAC I need help adjusting it. I assumed the IAC was in the correct position when I replaced it, but I'm suspect the PO had adjusted it when some of these parts I replaced went bad or possibly when they plated off the EGR (I replaced the egr and cleaned the egr tube) and now I find myself trying to adjust the IAC. I've adjusted it twice. The first time I was very close, but was not perfect so I adjusted it a turn more and it's acting up again. I've ready your very helpful advice, but call me slow I could use a bit more help or even pictures on how to do it the way you describe. The adjustments I made were before I found this page and I was told to just unplug it adjust it to 3500 rpm turn it off, plug it in, start it up for a minute then reset the ECU and I'd be good to go. Which is "close" to what I read here but miles away too. It did make it much better, but one more turn and it's back stalling out again. Can anyone give me pictures or better yet a video? I've hardly been able to drive the jeep since I purchased it. 

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Nate...You're rightfully frustrated if the Jeep has not been roadworthy...I'm guessing you read through the ISA adjustment procedures in the extensive 2.5L TBI coverage here at the forums.  Here is a quick recap in the PDFs:

Here is the factory ISA motor adjustment procedure.  Use great care when running the engine speed up.  Stay away from the engine's cooling fan!  (One way to eliminate risk of fan injury during this adjustment would be to temporarily remove the drive belt.):

2.5L TBI ISA Motor Adjustment.pdf

Read these additional details on Closed Throttle (Idle) Switch testing in conjunction with the ISA motor and adjustments before performing ISA motor adjustment and tests:

Closed Throttle Idle Switch & ISA Testing.pdf

Note that I suggested a Closed Throttle (Idle) Switch test alongside the ISA Motor adjustment.  There is a need to recycle the ISA if you adjust it incorrectly.

When you read the ISA steps, the plunger needs to be actuated with the actuator tool before you set to 3500 rpm.  Again, stay away from the fan or disconnect the fan belt to eliminate risk of injury with the engine running at this relatively high speed.  Don't lean over the fan!  If you are not actuating the plunger with the ISA connector uncoupled, you will not be able to get an accurate setting of 3500 rpm.

I doubt you have access to the "ele. AB.99" tool.   Here is the link to my 2.5L TBI troubleshooting article at the magazine, which helps explain the relationship between the various devices and symptoms associated with problems:  http://www.4wdmechanix.com/Tuning-and-Troubleshooting-the-2.5L-Jeep-TBI-Four?r=1

There is a close relationship with the Closed Throttle Idle Switch, the ISA Motor and the TPS.  Read through my article, it's well illustrated.  See if any one of these three devices could be causing your idle speed trouble.

We can resume the troubleshooting process once you have reviewed the information...


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Nate...On your behalf and that of many other Jeep 2.5L TBI troubleshooters, I put together a comprehensive list of PDFs covering the troubleshooting of 2.5L TBI, diagnostics and adjustments like you're attempting.  Here is the forum page:  

You will find information in the first PDF on the list that thoroughly covers the ISA motor and closed throttle switch, including wiring diagrams and diagnostics points.  The ECU pinout shows the pins for extending and retracting the ISA motor.  There is also a clear illustration of the ISA motor/closed switch connector.  If you follow the wiring diagram that I also included for your "Model 81" Jeep YJ Wrangler, you can find the pin sockets at the ISA motor connector for the ground, the hot and the retracted or extended positions of the plunger.

Read the PDF that covers adjustment of the ISA/closed switch and references to how far the plunger should extend and stop.  You can test the ISA/closed switch this way.  The PDFs are actual factory testing and adjustment procedures.  

You can simulate the "ele. AB.99" tool's functions by disconnecting and powering up the ISA connector directly.  Determine the ground, hot, retract and extend pins.  If you make a battery jumper, be sure to fuse the jumper to protect the ISA motor and wiring.  Avoid playing with the ECU connector other than to trace wire color codes back to the ISA connector.


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