puzzling idle
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Joined: Sep 2019
Posts: 3
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Model: Cherokee (XJ)
puzzling idle
replaced motor in my 97 jeep..4.0/6 cylinder...idle surges from 750 to 1200...without cost I replaced tps...iac...and map sensors...no change in idle....6 new plugs...no change...replaced injector rail and injectors...no change...checked for vaccum leak with can of carb cleaner...no change....cap..rotor..and wires look great....Im at wits end and hoping for some constructive insight
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Joined: Feb 2019
Posts: 1,359
Likes: 169
From: NC
Year: 2004
Model: Grand Cherokee (WJ)
Engine: 4.0
Whenever you replace an electronic sensor such as the IAC valve etc... You are supposed to disconnect the battery and reset the main ECU-PCM after final installation. I would think you probably did disconnect the battery prior to removing any of those sensors when you were replacing them, but maybe not? Even if you did, I would go ahead and disconnect the battery cables one more time, and touch the terminal ends together to reboot the PCM a final time now that installation of these sensors is completed. It may not be the reason for your Jeep having the surging idle, but it is the proper procedure...
Did you thoroughly clean the hole that the IAC valve plunger goes into, as well as the throttle body itself? If you just stuck a new IAC into a dirty passage, it will most likely still idle incorrectly if it is caked-up and sticky with deposits. The IAC has a seperate housing assembly which attaches to the throttle body via special hollow star-pointed bolts. I would pull off the IAC housing assembly to clean up the passageway where the pintle of the IAC plunger slides in and out.. This area is probably the most important area to thoroughly clean up for where the IAC valves proper functionality is concerned, for your idle.
There is supposed to be an o-ring at the very base of the IAC valve too, where the stepper motor mates up to the IAC housing. I am not sure if the newer IAC valve you used as a replacement would of had this o-ring or not, or if you noticed it from off the old one to transfer it over onto your replacement IAC valve? Also, the o-ring could be old and chewed up now possibly, and not sealing things up here to do its job correctly anymore too?
The IAC is controlled by the ECU taking inputs from the Cam/crank sensors for RPM and from the temperature sensor.
I would scrutinize at the IAC valve. Something is not right with your idle, and the IAC valve is what mostly controls your idle. You can also pull off the IAC valve housing with the IAC valve stepper motor still in-place inside of its housing, to test it for having proper functioning while it is removed outside of the vehicle and on the bench. Apply 12Volts to it and see what it does.. see if it is performing normaly inside of the housing as it should.
You can use a multimeter and test things. You can test the wires leading to the IAC valve and see if they are delivering the proper steady electricity to the IAC valve, and at the IAC valve itself for having the proper electricity too.
Maybe it could be something else? Maybe a fueling issue? I do not think it is though. It sounds more likely to be a dysfunction at, or with the IAC.
Did you thoroughly clean the hole that the IAC valve plunger goes into, as well as the throttle body itself? If you just stuck a new IAC into a dirty passage, it will most likely still idle incorrectly if it is caked-up and sticky with deposits. The IAC has a seperate housing assembly which attaches to the throttle body via special hollow star-pointed bolts. I would pull off the IAC housing assembly to clean up the passageway where the pintle of the IAC plunger slides in and out.. This area is probably the most important area to thoroughly clean up for where the IAC valves proper functionality is concerned, for your idle.
There is supposed to be an o-ring at the very base of the IAC valve too, where the stepper motor mates up to the IAC housing. I am not sure if the newer IAC valve you used as a replacement would of had this o-ring or not, or if you noticed it from off the old one to transfer it over onto your replacement IAC valve? Also, the o-ring could be old and chewed up now possibly, and not sealing things up here to do its job correctly anymore too?
The IAC is controlled by the ECU taking inputs from the Cam/crank sensors for RPM and from the temperature sensor.
I would scrutinize at the IAC valve. Something is not right with your idle, and the IAC valve is what mostly controls your idle. You can also pull off the IAC valve housing with the IAC valve stepper motor still in-place inside of its housing, to test it for having proper functioning while it is removed outside of the vehicle and on the bench. Apply 12Volts to it and see what it does.. see if it is performing normaly inside of the housing as it should.
You can use a multimeter and test things. You can test the wires leading to the IAC valve and see if they are delivering the proper steady electricity to the IAC valve, and at the IAC valve itself for having the proper electricity too.
Maybe it could be something else? Maybe a fueling issue? I do not think it is though. It sounds more likely to be a dysfunction at, or with the IAC.
CF Veteran
Joined: Nov 2017
Posts: 3,700
Likes: 238
From: Groton, MA
Year: 1995
Model: Cherokee(XJ)
Engine: I6 4.0L
Whenever you replace an electronic sensor such as the IAC valve etc... You are supposed to disconnect the battery and reset the main ECU-PCM after final installation. I would think you probably did disconnect the battery prior to removing any of those sensors when you were replacing them, but maybe not? Even if you did, I would go ahead and disconnect the battery cables one more time, and touch the terminal ends together to reboot the PCM a final time now that installation of these sensors is completed. It may not be the reason for your Jeep having the surging idle, but it is the proper procedure...
Did you thoroughly clean the hole that the IAC valve plunger goes into, as well as the throttle body itself? If you just stuck a new IAC into a dirty passage, it will most likely still idle incorrectly if it is caked-up and sticky with deposits. The IAC has a seperate housing assembly which attaches to the throttle body via special hollow star-pointed bolts. I would pull off the IAC housing assembly to clean up the passageway where the pintle of the IAC plunger slides in and out.. This area is probably the most important area to thoroughly clean up for where the IAC valves proper functionality is concerned, for your idle.
There is supposed to be an o-ring at the very base of the IAC valve too, where the stepper motor mates up to the IAC housing. I am not sure if the newer IAC valve you used as a replacement would of had this o-ring or not, or if you noticed it from off the old one to transfer it over onto your replacement IAC valve? Also, the o-ring could be old and chewed up now possibly, and not sealing things up here to do its job correctly anymore too?
The IAC is controlled by the ECU taking inputs from the Cam/crank sensors for RPM and from the temperature sensor.
I would scrutinize at the IAC valve. Something is not right with your idle, and the IAC valve is what mostly controls your idle. You can also pull off the IAC valve housing with the IAC valve stepper motor still in-place inside of its housing, to test it for having proper functioning while it is removed outside of the vehicle and on the bench. Apply 12Volts to it and see what it does.. see if it is performing normaly inside of the housing as it should.
You can use a multimeter and test things. You can test the wires leading to the IAC valve and see if they are delivering the proper steady electricity to the IAC valve, and at the IAC valve itself for having the proper electricity too.
Maybe it could be something else? Maybe a fueling issue? I do not think it is though. It sounds more likely to be a dysfunction at, or with the IAC.
Did you thoroughly clean the hole that the IAC valve plunger goes into, as well as the throttle body itself? If you just stuck a new IAC into a dirty passage, it will most likely still idle incorrectly if it is caked-up and sticky with deposits. The IAC has a seperate housing assembly which attaches to the throttle body via special hollow star-pointed bolts. I would pull off the IAC housing assembly to clean up the passageway where the pintle of the IAC plunger slides in and out.. This area is probably the most important area to thoroughly clean up for where the IAC valves proper functionality is concerned, for your idle.
There is supposed to be an o-ring at the very base of the IAC valve too, where the stepper motor mates up to the IAC housing. I am not sure if the newer IAC valve you used as a replacement would of had this o-ring or not, or if you noticed it from off the old one to transfer it over onto your replacement IAC valve? Also, the o-ring could be old and chewed up now possibly, and not sealing things up here to do its job correctly anymore too?
The IAC is controlled by the ECU taking inputs from the Cam/crank sensors for RPM and from the temperature sensor.
I would scrutinize at the IAC valve. Something is not right with your idle, and the IAC valve is what mostly controls your idle. You can also pull off the IAC valve housing with the IAC valve stepper motor still in-place inside of its housing, to test it for having proper functioning while it is removed outside of the vehicle and on the bench. Apply 12Volts to it and see what it does.. see if it is performing normaly inside of the housing as it should.
You can use a multimeter and test things. You can test the wires leading to the IAC valve and see if they are delivering the proper steady electricity to the IAC valve, and at the IAC valve itself for having the proper electricity too.
Maybe it could be something else? Maybe a fueling issue? I do not think it is though. It sounds more likely to be a dysfunction at, or with the IAC.
Thread Starter
Newbie
Joined: Sep 2019
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Model: Cherokee (XJ)
puzzling idle
I did reset the computer...I even changed the computer out with a known good one....pulled the iac off and cleaned it again...no change....Ive got a obd2 hand scanner Ive hook up several times...no codes...but when I go into live data mode it will say linking error???...…..The one time it did connect to live data the advance would show 5 lines at 11/12 degrees and one line at 2/3 degrees....still puzzled
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Joined: Feb 2019
Posts: 1,359
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From: NC
Year: 2004
Model: Grand Cherokee (WJ)
Engine: 4.0
These sensors can be puzzling. It sounded like IAC. Seems its okay.. What else?
It may be your CPS (Crank Position Sensor). There are three reasons which make me think it may be something with the crank sensor. It is verified to not ussually give you a DTC (Diagnostic Trouble Code), or CEL (Check Engine Light)... A faulty CPS symptoms can resemble about anything. A rough idle has been proven to result from having a faulty crank sensor... Also, when you have a faulty CPS, there is reliable evidence it causes scanners to be unable to communicate, or link-up when at the data port (DLC).
I would unplug the CPS sensor from at where it runs up into the engine bay; the pigtail harness, its connector connecting it into the main electrical wiring harness. Prior to disconnecting the CPS, I would pull the battery cables. After unplugging the CPS, I would consider touching the battery cable terminals together again to reboot the main ECU-PCM, before I went back to the Data Link Connector (DLC) with the scanner. If it is the CPS causing your poor idle performance, this could prove it. By removing the faulty CPS part out of the circuit, you will see it... The scanner will now be able to communicate, linking up to the active live data feed on your OBDII device.
You can test the CPS too. It is a hall-effect. So, there is no reliably correct way to test it in Ohms. There is a way to get around this fact though: Instead of testing within the reccomended resistance range. You set it ridiculously high on the meter. This will allow you to prong the 3-pin CPS connector with a DVOM, and fool the device into giving you a reading of in resistance.. It may be a fluctuating number, and is not a fully reliable test.
It may be your CPS (Crank Position Sensor). There are three reasons which make me think it may be something with the crank sensor. It is verified to not ussually give you a DTC (Diagnostic Trouble Code), or CEL (Check Engine Light)... A faulty CPS symptoms can resemble about anything. A rough idle has been proven to result from having a faulty crank sensor... Also, when you have a faulty CPS, there is reliable evidence it causes scanners to be unable to communicate, or link-up when at the data port (DLC).
I would unplug the CPS sensor from at where it runs up into the engine bay; the pigtail harness, its connector connecting it into the main electrical wiring harness. Prior to disconnecting the CPS, I would pull the battery cables. After unplugging the CPS, I would consider touching the battery cable terminals together again to reboot the main ECU-PCM, before I went back to the Data Link Connector (DLC) with the scanner. If it is the CPS causing your poor idle performance, this could prove it. By removing the faulty CPS part out of the circuit, you will see it... The scanner will now be able to communicate, linking up to the active live data feed on your OBDII device.
You can test the CPS too. It is a hall-effect. So, there is no reliably correct way to test it in Ohms. There is a way to get around this fact though: Instead of testing within the reccomended resistance range. You set it ridiculously high on the meter. This will allow you to prong the 3-pin CPS connector with a DVOM, and fool the device into giving you a reading of in resistance.. It may be a fluctuating number, and is not a fully reliable test.
Last edited by Noah911; Sep 22, 2019 at 02:03 PM.
CF Veteran
Joined: Dec 2015
Posts: 2,139
Likes: 93
From: Syracuse, NY
Year: 1996
Model: Cherokee (XJ)
Engine: 4.0L
You can test the CPS too. It is a hall-effect. So, there is no reliably correct way to test it in Ohms. There is a way to get around this fact though: Instead of testing within the reccomended resistance range. You set it ridiculously high on the meter. This will allow you to prong the 3-pin CPS connector with a DVOM, and fool the device into giving you a reading of in resistance.. It may be a fluctuating number, and is not a fully reliable test.
A multimeter can -not- test the signal being generated by the sensor. So a weak or defective magnet/coil in the sensor or a damaged flex-plate/fly wheel (teeth missing, corroded, foreign debris). Or timing disagreement between the cam pickup sensor and crankshaft position sensor (they have a fixed relationship with each other via the timing chain) These things can only be seen with an oscilloscope.
Most people do what they can with the multimeter, and then parts swap and see if it helps. This is a sensor that is good to have a known-good (Mopar) spare for.
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Joined: Feb 2019
Posts: 1,359
Likes: 169
From: NC
Year: 2004
Model: Grand Cherokee (WJ)
Engine: 4.0
A multimeter can test the wiring/connectivity of the sensor (to rule out opens/shorts in the connector and wiring).
A multimeter can -not- test the signal being generated by the sensor. So a weak or defective magnet/coil in the sensor or a damaged flex-plate/fly wheel (teeth missing, corroded, foreign debris). Or timing disagreement between the cam pickup sensor and crankshaft position sensor (they have a fixed relationship with each other via the timing chain) These things can only be seen with an oscilloscope.
Most people do what they can with the multimeter, and then parts swap and see if it helps. This is a sensor that is good to have a known-good (Mopar) spare for.
A multimeter can -not- test the signal being generated by the sensor. So a weak or defective magnet/coil in the sensor or a damaged flex-plate/fly wheel (teeth missing, corroded, foreign debris). Or timing disagreement between the cam pickup sensor and crankshaft position sensor (they have a fixed relationship with each other via the timing chain) These things can only be seen with an oscilloscope.
Most people do what they can with the multimeter, and then parts swap and see if it helps. This is a sensor that is good to have a known-good (Mopar) spare for.
There is a way to test the CPS, to verify seeing if it is generating the proper electrical signal. It produces an intermittent 5Volt signal which you can see on a DVOM while manually hand cranking the engine.
You can check proper ground and power at the CPS with a DVOM. It is possible to get a reading in resistance - Ohms, at the CPS too. It can be obtained in a round about way. It is not considered to be a reliable or accurate reading, since the CPS is a hall-effect type sensor. it is still an obtainable value though.
CF Veteran
Joined: Nov 2015
Posts: 2,856
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From: syracuse ny
Year: 1997
Model: Grand Cherokee (ZJ)
Engine: 4.0, new lifters valve job with new springs and exhaust valves, preload set with shims
Parts cannon got fired a lot on this one. Stand clear! Ready, don't aim, fire!!
Um how about checking for codes just a passing thought
Um how about checking for codes just a passing thought
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Joined: Sep 2019
Posts: 3
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Model: Cherokee (XJ)
puzzling idle
thanx for the input...sent me in the right direction....turns out that the man who helped me r/r the motors removed the crank sensor bolt thinking it was a bell bolt and didn't replace it...so the crank sensor just hopped around giving me a surging idle...again thanx for the input
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