MAP Sensor and Idle Speed Stepper Motor

First off what does the Idle Speed Stepper Motor do anyhow? How does it regulate the idle speed?
My '88 4.0 is having a tough time idling and accelerating. I took it out last night and when I went to pull out from the driveway the Jeep seemed real sluggish and then it got over it and started to run smooth for about 5 seconds but as soon as I added a little throttle it bogged down again. I had to barely touch the throttle to get it to go. I stopped in a gas station to take a look under the hood. I pulled out my air filter because it seemed like the engine was gasping for air. The air filter was dirty but not the cause of the gasping sound. The engine would run rough for about thirty seconds and make this sucking chest wound sound and then the sound would stop and the engine would run smooth for five seconds and then repeat over and over. It sounded to me that the engine was choking itself to death. I revved the engine a bit and that almost caused a backfire through the air box. It seems like it is getting too much fuel and not enough air. What controls the amount of air going into the manifold. Would that be the MAP sensor? Which one of these is the culprit or is it something else? Any help would be much appreciated.

I think the first thing I would is pull the throttle body and take it inside on a workbench, give it a really good cleaning, qtips and all, check and clean the electrical connections and go from there. Cleaning the TB out solves alot of ills just don't get to liberal with the carb/Fi cleaner, some are harmfull to plastic pieces.

It sounds to me that you have a bad TPS, especially if the idle is erratic and you're getting hesitation/bucking when you press the gas pedal. Disconnect the wiring harness altogether from the TPS. If the engine runs better, you've found the problem.

I have a 88 XJ and have done all the things I am going to suggest so don't have a heart attack. It what to be expected for a 16 year old vehicle.

sluggishness
This can be caused by an obstruction in the intake or exhaust system. You did not say how many miles on your XJ. At 125,000 I replaced the Cat because of a noisy heat shield. It was like taking a potato out of the tail pipe.

In the past year I had the tail pipe rust off my 6 year old muffer. I had it changed and spent the money on dealer parts. I wanted something that sounded nice so after 7-10 hours of driving I was not worn out by the sound. I drove it with the new muffler and it had more power and I gained 1 mpg around town and on the highway.

If your cat has broken up then it may of thrown trash in the muffler. I recommend you change out the cat, muffler and tail pipe. If your cat is 16 years old then it time to change. Also it a general practice to change the oxygen sensor when you change the cat, that way if the O2 sensor is bad it won't accelerate the failure of the new cat.

Irregular idle
There are 3 items that control idle - MAP sensor, TPS and Idle air controller. Like Rich said have the throttle body cleaned on a bench. If it is dirty it can plug the map sensor tube orifice, also the air passage for the IAC gets so dirty the stepper motor can not move correctly.

If those sensors are also 16 years old I'd change them all, they are worn out.

Googenheimer said:

First off what does the Idle Speed Stepper Motor do anyhow? How does it regulate the idle speed?
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the idle motor - often called an IAC - regulates idle by opening and closing an air valve.

In conjunction with your fuel injectors and O2 sensor feedback, if your idle is too slow, the IAC motor lets more air in and the injectors dump a corresponding increase in fuel, and your idle returns to normal.

If your idle gets too high, the IAC closes a little and less fuel is injected - causing idle to drop.

mysterious, are fuel injected vehicles that don't use an IAC (like the isuzu 2.6l).. those use elfin magic to regulate idle.

IAC is basicly a controlled vacum leak. Thats why you can put a rag in the throttle body, and the eng still runs. On 88-90 4.0, Tps is adjustable for idle,
just adj a 89 Commanche today. Adj is based upon in/out voltage.

Thanks for the advice. Here's a little update. The Jeep has a little trouble starting up but then revs smooth upon ignition and then runs for about 10-15 seconds gradually becoming worse and worse till it finally starts to thump pretty bad and sounds like it's dieseling a bit. It gets so bad that I don't dare let it run very long like that for fear that I'm going to blow something. I've pulled the TPS and it seems to test OK. I also pulled the Idle Stepper Motor and cleaned the tip of it off, it had a little carbon build-up on it. Now, what exactly would happen with a restricted exhaust? Could that be what is causing it to gradually worsen by building up too much back pressure? It does seem to breathe ok when it first starts up and I can't tell how it's doing after that seeing as how it comes out in big puffs every half second or so due to the weird thumping idle. I have yet to look in and clean the throttle body. I will do that next seeing as how I've already pulled off the TPS and the Idle Stepper Motor. Could this also be an EGR problem or would the EGR not affect it too adversely?

Well, now I've checked the throttle body and the EGR. The TB is now very clean and there were no restrictions. The EGR seems to test out OK and is not sticky. I'm going to try disconnecting the TPS just to double check that it is not the problem. I've also discovered that the "sucking chest wound" sound is the IAC. The noise gets louder as the IAC starts to close and then disappears when it opens and lets the air pass by this in turn results in the engine running smooth but at a high idle. When I blip the thottle the engine doesn't respond for a good half second or more and makes the engine lag and sometimes belch back out the intake. It will also not run under slight throttle pressure but will respond when I hold the throttle down. What would cause such a lag in engine response and why would it burp back through the intake. These problems seemed to point to the EGR but it's good, seals well, and disconnecting it didn't do anything for the bad idle. Also shouldn't the IAC hold constant instead of slowly closing and then opening etc. Any help or ideas?

I had a similiar problem with my 88', and after replacing the TPS, Air Idle Sensor, and trying someone else's MAP sensor we discovered a vacuum leak at the EGR that was causing the problem.

How did you discover the leak? I took mine off and sucked the air out of it. It seemed to move ok and it shut ok too. From what I have read on this board though it seems to me that all my symptoms point to a bad EGR. If I could just figure out why it's burping back up through the air intake and hesitating (when I blip the throttle it sounds like everything stops for just a half second and then resumes).

While you're at it, if you suspect egr problems, you might want to check the egr solenoid. I've heard that they sometimes burn out (literally - a hole in it), and this might make egr valve open at the wrong time. Have you tried just plugging or clamping off the vacuum hose to the egr valve? If the problem doesn't go away, then you'll knows it's not caused by the valve opening.

Have you also checked thoroughly for vacuum leaks and leaks in manifold or throttle body gaskets?