Noah911

Those test results actually look pretty good for the TPS. I was expecting to see something much worse. The 3.42 VOLTS at WOT is a little low. It should really be about 3.8 VOLTS or above (and below 4.49 VOLTS) at WOT. That reading should not be the issue though... At 3.5 VOLTS WOT people are not generally reporting on having poor running conditions. And your 0.55 VOLTS at idle looks perfectly fine. It has a good 5 VOLTS supply. The ground is less 100 mV like it should be too.

From the pictures you posted today that IAC valve looks pretty bad. I would go ahead and test the IAC. Try this first once everything is back together. Keep the IAC valve removed from its housing on the throttle body but, keep it connected to its wires like it normally would be otherwise. Watch the stepper motors plunger while you turn the ignition key to RUN. The IAC plunger of the stepper motor is supposed to move into a 'home' position whenever you put the key to the RUN position. It should be defaulted to about 1" with everything turned off and move in about 3/16th" with the key in the run position. This is a kind of crude test to see if the IAC works.

There should be 4 pins on the IAC. Probe the outer two pins for resistance, and then the inner two pins for resistance. These should read at 50 +/- ohms.

At minimum.. the IAC obviously looks like it needed a good cleaning.

Other thoughts thinking ahead are checking the Engine Coolant Temperature (ECT) sensor. I will stick with you so long as I have the time and feel like I am able to help... The reason being is the ECT sensor is what provides the PCM with crucial information necessary to properly adjust the air/fuel mixture. The ECT may be tested in various ways with a DVOM and an infrared thermometer, if you need to go there. It can also be cleaned up if it is coated in a sort of varnish from all of the years it has been in use.

I think it would also be a very good thing to just go ahead and check to make sure there are no vacuum leaks anywhere by spraying all around inside the engine compartment with it runnung while listening for a change in the engine RPM.

I will be curious if cleaning the IAC helps. It very well may... Also, if it moves to home when turning the ignition key to run, and if it tests okay for resistance?

Last thought right now. You do not absolutely need the IAC but, you do... What I mean is the Jeep should start and run without having the IAC connected. There have been other members who were having issues with high 800 - 1300 RPMs idle that fluctuated in various ways. These people left the IAC disconnected and started their Jeeps. For some of them, their erratic idle and high RPM problems totally disappeared for them by doing this. This was how they knew it was bad. So, they replaced their IAC after checking this.

Try to run it with the IAC disconnected and see what happens if problems persist..

Also, with the IAC connected back up and engine running. Turn the air conditioner on while watching the RPMs. The RPMs should temporarily drop once having the a/c turned on at first.. and then the RPMs should pick back up again real quickly if the IAC is working correctly, as another quick crude test.

(edited)
Minor errors corrected in case you read this before I re-read it myself.

philwarner

I cleaned the IAC with carb cleaner and wire brushes by hand and sprayed the pins and connector with contact cleaner, and blew them out. I sprayed carb cleaner in the IAC bore of the throttle body and wiped it out with Q tips, and reinstalled the IAC. I also sprayed the TPS connector with contact cleaner and let it dry before reconnecting. I stopped for lunch and then went out to start the engine again. When I turned on the key the warning lights came on for bulb check (Including the check engine and check gages) and then went off, but the check gages light came back on. The engine started first crank but “idled” very roughly between 1100 and 1300 RPM with the tach needle dipping periodically. The gas gage at first was at empty and the low fuel light came on and then it went off and then came on again and then off again and the gage eventually went up to 1/2. The check gages light stayed on so I plugged the scanner in while it was running and it said no “MIL” light and no DTCs set. I revved the engine to 2500 RPM twice and let is back down and it slowed and then died.

A little later I restarted the engine again (had to crank it awhile this time) and then used the accelerator to hold it around 2500 RPM and it still surged at about 1 second cycles, the check gages light stayed on, and the gas gage worked it’s way slowly up to just under 1/2. I took a 30 second video of the tachometer as I let it back to “idle” at 1500 RPM and the tach needle dipped several times. The 309 second .mov file was 15 MB and apparently is still too big to upload.

I just now saw your post with IAC checking suggestions. I may have to do those tomorrow, but I'll see what I can do now before wine time. Does the IAC housing just pull out of the throttle body once the two screws are removed? I'll go take them back out and try the IAC test to see if it moves with the key on. I have a disconnect on the battery negative so I can disconnect it, turn the key to on and then reconnect the battery while watching the IAC. This is getting almost as frustrating as the virus thing, especially when I thought the replacement PCM would solve all its problems.

Noah911

"Does the IAC housing just pull out of the throttle body once the two screws are removed?"

Yes, it pulls right out.

It is obviously only frustrating because you have not found the problem yet. Despite the fact that I am sure you are probably not a mechanic, and I certainly am not, and you are receiving help remotely from people who you do not know to fully trust.. you have actually made good progress so far I think.

It starts and runs now. It sounds like it probably definitely did have a faulty PCM needing to be replaced. I wonder what the reason is for why your Jeep is throwing a CEL when your scanner shows no codes?

There are a lot of possible reasons for those problems.. and you have moved into the stage of either ruling some of those possibilities out, and/or finding the fault(s). It appears the TPS has been temporarily ruled out based on the test results. The IAC is being investigated and has been serviced (cleaned).

The CkPS sounds like it needs tested. That gas gauge problem, and the scanner not reading a code (CEL, but the scanner is not seeing it...) Both of these are signs of a CkPS type of failure, the long cranks and stalling out etc...

O2 sensor wires with a short will cause CkPS faults too, since they are tied into its circuitry through the ASD.

philwarner

Oops! I removed the IAC and measured between the collar on the body and the underside of the tip at .490, turned the key on with the battery disconnected and then connected the battery and the plunger moved to .730 between body collar and tip underside. I disconnected the battery and it did not move back and when I reconnected the battery it moved again and spit the tip out.

So now, is there a way to reverse the stepper motor to get the pintel back to its at rest position?




when I was checking things yesterday and watching items on the scanner data stream, the coolant temp rose from 15 degrees C to 23 degrees C (approx 60 F to 75F) while the engine was running just before it died and would not restart, so I assume the coolant temp sensor is working and the OBD2 system is reading it. The IAC housing appears to be a part of the 5.2 throttle body and not a separate piece as far as I can see.

Noah911

I do not want to bombard you with information. I do want to help though. You can always private message me if at anytime I have not responded to your thread for some reason. I pretty much almost always check this site daily though. I just worry maybe my focus is tunneled in some way. If I should step back to let others take over with a different perspective?

There can sometimes potentially be what seems like a lot to check while hunting for fault(s). One thing at a time though.. Focus on a certain aspect and keep moving forward until the picture begins to hopefully come clear.

Noah911

Whoa.. I think you just found at least part of your problem man. That IAC looks like it needs replaced!

What does everyone else think?

philwarner

There was no CEL before except during the temporary bulb check, but now with the IAC out there is a check engine as well as the check gages light and if I start the engine it races at 2500 RPM. My scanner could not communicate with the original PCM, so the fact that it can communicate with this one is a plus of sorts, and this PCM is producing the 5 volts for the sensors where the old one was not, so that is a step in the right direction too, But now that I have made it even worse I think it is definitely wine time before I screw something else up. Tomorrow is another day.

In the mean time if anyone knows which pins and what voltage and polarity is needed to reverse the IAC stepper, motor please chime in.

Noah911

The plunger of the IAC is not supposef to fall apart like that when energized. This looks like it was a big part of your idle problem (the IAC). It correlates well with the symptoms you have been having, and looks like it was needing to be replaced... The plunger should normally return on its own to whatever position it is supposed to be in when you turn the power on/off.

Ultimately, the IAC is not supposed to fall apart like that when energized. This indicates the IAC was mechanically faulty.

Noah911

I was thinking you were saying when you ran your Jeep the IAC was fully installed except it was not electrically hooked up. I took out a paragraph from my last post above because of this.

philwarner

I decided to forgo wine time and went to town where I bought new IAC at O’Reillys for $34.71 and it measured 3.78 from body collar to the underside of the pintle out of the box. I installed it and started the engine and was back to a rough surging 1500 RPM “idle” and the tachometer needle blipping, exactly like it was before I made the old IAC self destruct trying to test its activation outside the throttle body. The check engine light did go off after the new IAC was connected, also just as before.

Looking at the little pintle and at a IAC wiring diagram (below) I assume that when activated the stepper motor moves the pintle out until it is stopped by its seat to establish the closed position and then reverses and opens it the number of steps its little brain has been programed to do for a cold idle. In my case since there was nothing to stop the pintle it didn't know the closed position and when I activated it a second time it tried to move out again until it just spit the pintle out. I haven't found any source that says whether this is the way it works, the discussions just say the ECU or PCM adjusts it but not how it determines a closed or open position, but it seems logical that it must find the closed position first and then back off however many steps it has been programmed to use. I am pretty sure my old IAC was OK before I made it self destruct. Perhaps if I back probe the connector I can find out what voltage turns the stepper motor to get it to retract.

Many sources say a fluctuating idle is a symptom of a bad IAC, but now it is new and it is still surging at high idle. I'll check the vacuum next and look for vacuum leaks, and perhaps pull the throttle body for a cleaning. I also want to find out if the PCM feeds the tachometer and what might be causing it to keep dipping, and the erratic fuel gage is another mystery. I am thinking that used PCMs are a bit of a crap shoot and this one may well have problems of its own since I didn't have the idle, tachometer, and fuel gage problems before the other PCM died.

Noah911

I would not want to buy another PCM and find out the current PCM in there was probably actual a good working unit. As this would really dampen my spirits to the whole thing.

I would continue looking for any other possible reasons. I apologize if it was my fault for the IAC shooting the pintle out like that. It happened, and now there is a new IAC.

The first thing I would do today is look for any vacuum leaks.

philwarner

I found information from Autozone this morning which is a more complete description of IAC operation and confirms that if the PCM loses its memory of the IAC location, it will extend the pintle to the closed position to zero the counter and then adjust from there. I measured the resistance among pins on my original IAC and found 53 ohms between the two outer pins and 53 ohms between the inner two pins and no connection between the two sets. I tried putting 12 volts across the outer pins and nothing moved; the description below indicates the PCM may have to energize both windings at the same time to drive the pintle more than one step, but is not specific about that.

Another source suggests wiping the PCM memory by shorting both battery cables together (away from the battery) so I am doing that now, but I have to think that disconnecting the PCM entirely might do the same thing and account for the replacement PCM trying to find the IAC closed position even though it should have found it the first time the ignition was turned on with the old IAC mounted in the throttle body, unless the PCM loses memory every time the battery negative is disconnected or if this PCM does have other problems.

Here's the Autozone IAC information - the bold type is mine to highlight the relevant info in my notes. At the top of the web site page it says 99 to 05 Grand Cherokee but further down in large type it says 96 and in a section on specific removal directions it shows a 4.0 engine but does not have a 5.2 V8. Still, I think the operations description is correct for my 5.2. Now I just need to figure out which pins and what polarity to use to turn the little sucker to pull the pintle back in.


https://www.autozone.com/repairinfo/...96b43f802d7ae0

The IAC stepper motor is mounted to the throttle body, and regulates the amount of air bypassing the control of the throttle plate. As engine loads and ambient temperatures change, engine rpm changes. A pintle on the IAC stepper motor protrudes into a passage in the throttle body, controlling airflow through the passage. The IAC is controlled by the Powertrain Control Module (PCM) to maintain the target engine idle speed.

At idle, engine speed can be increased by retracting the IAC motor pintle and allowing more air to pass through the port, or it can be decreased by restricting the passage with the pintle and diminishing the amount of air bypassing the throttle plate. The IAC is called a stepper motor because it is moved (rotated) in steps, or increments. Opening the IAC opens an air passage around the throttle blade, which increases RPM.

The PCM uses the IAC motor to control idle speed (along with timing) and to reach a desired MAP during decel (keep engine from stalling).

The IAC motor has 4 wires with 4 circuits. Two of the wires are for 12 volts and ground to supply electrical current to the motor windings to operate the stepper motor in one direction. The other 2 wires are also for 12 volts and ground to supply electrical current to operate the stepper motor in the opposite direction.

To make the IAC go in the opposite direction, the PCM just reverses polarity on both windings. If only 1 wire is open, the IAC can only be moved 1 step (increment) in either direction. To keep the IAC motor in position when no movement is needed, the PCM will energize both windings at the same time. This locks the IAC motor in place.

In the IAC motor system, the PCM will count every step that the motor is moved. This allows the PCM to determine the motor pintle position. If the memory is cleared, the PCM no longer knows the position of the pintle. So at the first key ON, the PCM drives the IAC motor closed, regardless of where it was before. This zeros the counter. From this point the PCM will back out the IAC motor and keep track of its position again.

When engine rpm is above idle speed, the IAC is used for the following:



Off-idle dashpot (throttle blade will close quickly but idle speed will not stop quickly)



Deceleration airflow control



A/C compressor load control (also opens the passage slightly before the compressor is engaged so that the engine rpm does not dip down when the compressor engages)



Power steering load control



The PCM can control polarity of the circuit to control direction of the stepper motor.




IAC Stepper Motor Program

The PCM is also equipped with a memory program that records the number of steps the IAC stepper motor most recently advanced to during a certain set of parameters. For example: The PCM was attempting to maintain a 1000 rpm target during a cold start-up cycle. The last recorded number of steps for that may have been 125. That value would be recorded in the memory cell so that the next time the PCM recognizes the identical conditions; the PCM recalls that 125 steps were required to maintain the target. This program allows for greater customer satisfaction due to greater control of engine idle.

Another function of the memory program, which occurs when the power steering switch (if equipped), or the A/C request circuit, requires that the IAC stepper motor control engine rpm, is the recording of the last targeted steps into the memory cell. The PCM can anticipate A/C compressor loads. This is accomplished by delaying compressor operation for approximately 0.5 seconds until the PCM moves the IAC stepper motor to the recorded steps that were loaded into the memory cell. Using this program helps eliminate idle-quality changes as loads change. Finally, the PCM incorporates a No-Load engine speed limiter of approximately 1800 - 2000 rpm, when it recognizes that the TPS is indicating an idle signal and IAC motor cannot maintain engine idle. A (factory adjusted) set screw is used to mechanically limit the position of the throttle body throttle plate. The IAC motor through the PCM controls all idle speed functions.

philwarner

Brief update:I had disconnected the battery cables and connected the two together away from the battery as one source suggested to wipe the PCM’s memory of IAC position. I reconnected the battery cables with my negative cable disconnect open and then connected the old IAC with a clamp on it to limit the pintle travel and turned the key to on. When I closed the negative battery disconnect, the IAC vibrated and then withdrew the pintle. I reset the clamp and did it again and it vibrated and the pintle withdrew again to .275 between the body lip and the underside of the head or about .970 from the mounting flange to the tip of the pintle.
I decided to bite the bullet and pull the throttle body for cleaning. Maybe I can return the new IAC?

Noah911

I have difficulty trying to link URLs onto threads. If you search 'IAC Live training', youtube, there is an informative video on the IAC you might enjoy. It is about 20min and goes into great detail on a fully dissembled unit. It is for GM and Chrysler systems.

I looked up a lot on the IAC last night. What happened to yours confused me as to why it happened? Although, I did find the same result was reported by some other people as well. It makes sense with the PCM memory, and for how the IAC is designed. It was recommended to keep some pressure applied to the end of the pintle in order to prevent...

I do not know if this is true or not? I cannot recall where the information came from, or if it is specific to our Jeeps? I read in order for the PCM to fully re-learn the IAC for a full recalibration, the vehicle (Jeep?) needs one run-up.. to 35mph. This is probably only required to calibrate everything generally to normal overall driving, and not for the idle (or finding its end point).

With the IAC reinstalled, have you attempted to run the Jeep with the IAC electrical connector disconnected.. to see if there is a difference? If I was around my Jeeps, I would check on this myself by disconnecting it. Only just because I am now curious about this.

The readings of 53 ohms is perfect. There should be no more than a 3 ohms difference between the two measurements, and yours is spot on.

Noah911

I missed your last post. That was smart!