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spanky 03-10-2008 01:22 AM

rough idle
when i first start my jeep in the morning, it has a very rough erratic idle, for maybe 10-15 seconds, then it smooths out, and all is fine.

any suggestions why this might be happening?

Jerry Bransford 03-10-2008 10:16 AM

A rough idle is normally caused by either a dirty IAC (idle air controller) or a bad (dirty internally) TPS (throttle position sensor). Both are bolted onto the throttle body. You can remove the IAC and clean it with some throttle body cleaner and a brush or rag in ten minutes max. It has a solenoid actuated plunger that gets gunked up with black crud, and the same with the small hole in the throttle body the plunger fits into which is where the engine gets all of its air at idle. Use a baby bottle nipple brush to clean that small hole the plunger fits into. Just be gentle with the IAC's solenoid actuated plunger, don't turn it or pull/push on it. The IAC is located on the far side of the throttle body as you stand looking at the throttle body from the driver's side fenderwell. It is the sensor nearest to the valve cover.

The TPS is what tells the engine computer what position the accelerator pedal is at so if it's dirty inside, it can also cause an erratic idle. The TPS is sealed however so when it goes bad, it can only be replaced. The TPS is on the firewall side of the throttle body, opposite to the throttle control lever which controls it via the butterfly valve pivot shaft.

Once you're done cleaning/replacing either one, be sure to clean the inside of the throttle body with the remainder of the throttle body cleaner using the directions on the can. It get gunked up inside in normal driving.

slf41002 03-10-2008 10:48 AM

It could also just need a tune up.I see it is a 2000 how long ago was the last tune up and does it have a check engine light on or anything?I would go with Jerry as far as cleaning the IAC first then if that doesn't help I would check the plugs.On second thought check the plugs first cause the more I think about it it only does this for the first 15 secs I would say that you have one or more plugs that takes a few seconds the start burning the fuel in the cylinder.

Benjammin 03-10-2008 12:13 PM

I recently discovered that my Jeep was having the same problem only it lasts a lot longer than 15 seconds, it even happens after my Jeep is all warmed up. It's very rough when idling but when i step on the gas the vibration goes away. I figured it was due to the idling speed being too high or low or just the cold weather. I don't know too much about fixing vehicles or even determining whats wrong with them, I was hoping to get some insight before I took it to a mechanic so I don't sound like a complete moron.

UNLIMITLESS 03-10-2008 12:15 PM

I would check/clean the IAC. Doesnt take much to cause a problem.

spanky 03-10-2008 03:19 PM

jerry: good ideas. i will give this a shot.

since i just recently bought this thing, ive been meaning to go over stuff like this. just no time when i get off work, its too cold and too dark. stuff like this has to wait for the weekend, and this last weekend was spent working on my yota.

i will also just change out the plugs regardless. no clue knowing how long they been in there.

when i finally get to it, i will let ya know if this solved the idle problem.

thanks all!

spanky 03-12-2008 11:00 PM

ok so i managed to clean out the IAC and the entire throttle body. i didnt touch the TPS yet. just wanted to do one thing at a time, only because i am curious as to whats causing this.
anyways, i fired it up this morning, with the same erratic idle... BUT instead of the 10-15 seconds, it only lasts maybe 5 seconds tops.
i already bought new plugs, so i will do that this weekend, and if that doesnt solve it, then i will replace the TPS.
and if that doesnt do it, then im sure you will be hearing from me again! haha

slf41002 03-13-2008 03:19 AM

Let us know if the plugs take care of your problem.I am not a betting man but if I was I would say it solves the problem.

spanky 04-08-2008 12:20 AM


i did a seafoam treatment and then changed the plugs. this helped but im not 100% satisfied. it is running much smoother at idle and the throttle "feels" a little peppier.

guess it wont hurt to change the TPS now.

nater344 04-08-2008 04:25 PM

Spanky - my 97 does the same thing, but only every once in awhile. I have changed the plugs, wires, distributer cap, rotor, plugs, cleaned out the throttle body and IAC, and run fuel injector cleaner through it. It is much better now, but occationally acts up. I haven't replaced the TPS yet, but I am anxious to see if that solves the problem. Keep us updated.

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