replaced IAC and idle is now worse....

I have a 97 4.0L auto 4x4 that I recently replaced the IAC on with a brand new unit from Autozone. The Jeep has always had a bit of a rough idle nothing terrible for a Jeep but I thought it could be improved a little bit. I did start to have a small problem with starting and some said possibly IAC and some said TPS. Since I have replaced the TPS once in the past after it shorted out going through a mud hole, I decided to start with the IAC since it was the original one. Now it seemed fine when I first replaced it ran smooth and I was pretty pleased. But last night when it had been sitting a while and it had gotten below 50 degrees outside, it would run rough at idle even after warmed up to operating temps. I didn't notice it after I had left to head back home about an hour later. But then again this morning when I started it up it had a rough idle again. The rough idle lasted all the way to campus. After I had gone back out to the Jeep and hour later the rough idle had gone away. I am not sure where to start next so If somebody has an idea they want to throw out there I'd be willing to check/try anything.

Well, the IAC is a stepper motor that responds to commands from the ECU/PCM by extending or retracting to change the idle SPEED.

A rough idle wouldn't really be part of its job unless it is idling so low that it is about to stall.

What RPM is the engine idling at when fully warmed up--after about 20 minutes of operation?

When warmed up it usually idles around what I'm guessing to be 750-800 RPM since it sits right there around the first mark right under 1000 RPM on the tach.

Ben824 said:

When warmed up it usually idles around what I'm guessing to be 750-800 RPM since it sits right there around the first mark right under 1000 RPM on the tach.

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You have an auto trans, so that should be 700-750 RPM in DRIVE, fully warmed up.

Have you changed your plugs, wires, cap, and rotor recently? I had a REALLY rough idle on my 96 when I first bought it, I thought that was just how they were supposed to run. Replaced the above a few months ago and it was like night and day. My cap/rotor were horribly arc burned, looked like at least 50k+ miles on them, and the plugs were about 50% too wide gap-wise due to erosion.

kastein said:

Have you changed your plugs, wires, cap, and rotor recently.

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Agreed. Tuneup parts MUST be considered for any rough idle/driveability problems. Failure to do so can result in the chasing of ones tail.

birchlakeXJ said:

Agreed. Tuneup parts MUST be considered for any rough idle/driveability problems. Failure to do so can result in the chasing of ones tail.

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Which can be hilarious when your dog is doing it, but quite frustrating when you are doing it yourself.

I was actually thinking about checking all the tune up parts today. I bought that firepower ignition kit that is available from quadratec so I guess I just didn't think to check something like that but I'm sure that they are probably in need of replacing since it has been a few years and probably 40K miles since I last replaced those.

You need to clear out the ecu to relearn the IAC position

Wwildman said:

You need to clear out the ecu to relearn the IAC position

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And to do this I just unplug the Jeep's battery for a few mins right?

yup. Unplug it, remove it from the vehicle and put it on the other side of the garage, walk back to the jeep, and touch the two battery cables together for a few seconds.

You can skip moving the battery, but I want to be absolutely clear that you're touching the cables together... not the battery terminals... :explosion

Touching battery terminals together would involve a SPARKS show that can be quite dramatic.... much like arc welding....
Only differences would be watching the battery explode and you losing control of your bodily functions.....

Not a good idea.

Once upon a time, on a forum far, far away...

Actually, it was here on . Poster wanted to clear the PCM memory. Too lazy to pull BOTH cables from the battery's terminals. Took the shortcut of just removing one cable from one battery terminal and then touching it to the other cable that was still attached to the battery terminal.

He bought a new PCM.

That's interesting. Must have removed the cable attached to the negative terminal?

joe_peters said:

Once upon a time, on a forum far, far away...

Actually, it was here on . Poster wanted to clear the PCM memory. Too lazy to pull BOTH cables from the battery's terminals. Took the shortcut of just removing one cable from one battery terminal and then touching it to the other cable that was still attached to the battery terminal.

He bought a new PCM.

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StoneX said:

That's interesting. Must have removed the cable attached to the negative terminal?

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Yes.

kastein said:

yup. Unplug it, remove it from the vehicle and put it on the other side of the garage, walk back to the jeep, and touch the two battery cables together for a few seconds.

You can skip moving the battery, but I want to be absolutely clear that you're touching the cables together... not the battery terminals... :explosion

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Don't worry I'm not that much of a newb haha I would never touch the cables together while any wires or cables were still hooked to the battery. I will definitely be trying this later to day when I have some free time. Thanks for the help and I'll let yall know if it works.