4.0 idle stepper motor

Ok....

What the heck are you driving and what engine????

An 89 XJ does not have a stepper motor, that is an 89 4.2L YJ puppy, it has a throttle body with a TPS or throttle position sensor.

The TPS can be taken out and cleaned with carb cleaner. It is just your basic rheostat.

It can be tested with a multimeter too. If you have that, I can post the tests.

Mike

Homer Simps>

Try spraying the entire intake around the throttle body with a good carb cleaner like Berrymans, etc., until it shines like new. Particular attention should be paid to the stepper motor port on the side of the t-body. It's fairly common in modern engines for this port to crud up with carbon & gunk, fouling the stepper and making it stick.

Damn, that sounds expensive for the IAC motor! I think I gave $25 or $30 for mine when I swapped it out a couple of years ago (got it from Advanced Auto). Two things come to mind from that long hassle. First, remove the stepper from the throttle body and clean the shaft on the stepper well then get the entire bore it goes into. Take the inlet tube off and clean the opening from the top as well as from the side. Gunk builds up on the seat and down into the bore.

Next, on the top of the throttle body there is a hole, about 1/4 inch in diameter, on the fender side. That runs down to a plugged hole way down on the side. Under that plug is a jet. use a knife point or ice pick (remember those?) and pull the plug out. Turn the jet all the way in counting the turns so that you can get it back to where it was. Now, take the jet all the way out and clean that hole. It's an idle air bypass and runs all the way down into the mainfold. Mine was plugged solid and it took a lot of soaking and a long piano wire to get it cleaned out. Put the jet back in and adjust it to where it was. You can cover the access hole with silicone like the book says or just forget it. Clean the contacts in the IAC plug with WD40 and a stiff brush.

The IAC opens and closes to allow more or less air to pass instead of having to adjust the butterfly throttle plate t maintain an idle RPM. If it fails in an appropriate position to maintain a decent cold idle you see idle speed increase as the engine warms up or a high idle when you come to a stop. If it fails closed then cold starts will be a bitch and you will have to play with the throttle to keep it running once you do get it started. It also accounts for the higher cold start idle speed. I've rarely seen one cause stumbles on acceleration other than on a dead cold engine. The O2 sensor is a more likely candidate if it stumbles - or a plugged/bad MAP sensor. Clean the map tube and hole in the throttle body as well.

One caution: DO NOT turn the key on while the IAC is out and plugged in - it throws the shaft clear accross the garage and it's hell to get back together right!

Well according to my throttle body it does! it is an 89 XJ with a 4.0. The facotry service manual covers it off, not that that would mean anything!

Thx for the tips and it has been acting up daily so I should have some news to report tonight.

The IAC motor is in fact a stepper motor - uses phased excitation of the coils to rotate it - and it's gonna stay where it was when you pulled power off. I don't have the exact one handy at the moment, but on similar GM models you could depress the spring on the shaft to clear a locking ridge and manually twist the thing to set it. One of the precedures when changing those was to move it in far enough to prevent jamming it when you installed it. I think (but don't hold me to it) that you can do the same with the Jeep IAC. Just don't force it and break something! I'm going from memory here and a bunch of other steppers I've used in other applications, but you should be able to measure the resistance between pairs of pins or to ground to check for opens. Some motors reverse the polarity as well as the sequence in which they energize the coils to control direction, others use a grounded coil and just control the sequence.

The stepper moves the pointed end in and out of the hole it fits into about like adjusting an idle jet screw on an old carb - except the effect is to control the amount of air passing and not fuel. It isn't instantaneous but it's reasonably fast acting. The only way I know of to really test one without the proper control module is to unplug it and adjust it manually until it just closes off the port. Start the engine - you'll probably need to play with the accelerator to get it going - then open it one turn, try again until the engine idles smoothly at 800 RPM. Now try accelerating fairly slowly - should not stumble unless you punch it.

It took me almost 6 months to get my 88 MJ to idle right. I wound up pulling the throttle body and soaking it out in carb cleaner, replacing the IAC, the MAP sensor, and finally the O2 sensor. The cleaning did a lot of good, the MAP helped accleration (hesitation) all across the range, a new IAC stabilized the idle speed, and the O2 sensor got the idle smooth without dying at stop signs - finally - but it was a battle!

I agree with Will... The stepper motor just controls idle speed. The only way I can think of it causing stumble is if the idle was too low, but that would be obvious. Have you done all of the usual tune up items including plug wires? Joe, 90XJ

Oh ya...a couple of times already!

This AM I went and removed the airhorn. I covered the little air inlet section for the ISM and accelerated with no problems, it just idled slower. Which makes sense as the throttle is opening up and air is getting in. The ISM in an output only on the ECU so there is no direct feedback regarding its position.

Today I am thinking that my ISM operates correctly (or almost) as I have absolutely no problems when idling. The issue comes up when the throttle is open either while accelerating or when cruising. It always happens when the engine is hot, or the ambient temp is hot. It could be sitting in the sun all day and the engine is cold, and I have the problem after it starts.

The odd thing is is that it is immediately cured when I unplug the ISM. Like instantaneous. Will brought up a good point I should have remembered. Removing power to the ISM will not change its position as it is a stepper motor.

Today's thoughts are that the ISM is shorting out sometimes causing problems with the ECU, or my ECU is bad.

Decisions decisions...... $75 for a wrecker ECU or $120 for a new ISM......

I know it could be other things but I find it odd that disconnecting the ICM fixes it.

#^%#

Now you've illuminated something: that stepper pulls a significant current when it pulses and the stumble sounds like a MAP sensor. How about a bad ground for the computer/sensor grounds? Hit the IAC and screw up the other sensor readings with a shifting ground???

Just for grins, check the engine to firewall strap on both ends as well as the ground point inside up above the accelerator pedal.

thx for the insights....my new IAC doesn't make a difference...grrrrr....I hate guesswork mechanics! I will look at the MAP and ground strap and then off it will go to my mechanic. He is a great guy and often he will diagnose things for me and tell me how to fix it so I can learn.

This issue has gotten to the point of me becoming stubborn and wanting to figure it out myself!

Stay tuned...

Enjoy! I spent nearly 6 months after I bought the MJ dicking with the idle/off idle stuff. Like the Johnny Cash song "One Piece at a Time", I finally got it to idle and run smooth just off idle. Seemed like everything I did helped to some degree but there was always just a little more to do. I think that the last step for me was to replace the O2 sensor but I had replaced or tested everything else by then.

The dealer told me on one of the 88 XJs I work on, it is usually the TPS. Sure enough I changed the TPS and the idle went to normal and both the used and new ISS motor worked fine. later, dave

IIRC the ISS motor will be affected/adjusted/moved by info from the TPS and the ECM along with other sensors. AFAIK the TPS is usually the main problem. Here is a little info on the TPS;