EdW

Son just got his license recently and picked up a '98 Cherokee for his first car. Seemed to run fine initially, thought all was well. Then it would be idling fine and out of the blue get very rough, rpm's would drop and it would come close to stalling out. It would run like this for a few minutes and then recover and idle fine.

He was out on the road in traffic when this happened, and the car started to run very poorly, lost all sorts of engine power, so he tried to compensate by stepping on the gas, and it bucked resulting in a fender bender. I read that the Idle Air Control motor can do this so I took it out and cleaned it, put it back in, seemed to do the trick.

Next the water pump goes, so I install a new one today and while I have it idling, the same thing happens, smooth and then very rough out of the blue. This time I can clearly hear that the IAC motor has moved to limit the airflow into the throttle body and that's the reason for the poor idle. Took it for a ride myself then and it ran so poorly, I turned around after 100'.

Does anyone know how I can determine if the IAC is getting a false signal to limit the air flow or if the IAC itself is bad and doing this?

I should also add that I hooked up my SCAN tool to the Jeep as the check engine light was on, said that 02 sensor voltage was high bank 1 sensor 1 but the odd thing is that when I went to look at the waveform, I couldn't see anything. No other sensors were available to look at, just the Absolute Throttle Position. I doubt it's supposed to be this way as on other cars I can see Coolant Temp, Timing Advance, RPM, Intake Air Temp, etc.

Any thoughts on this would be VERY much appreciated as it really is a safety issue and the car is unusable as is because there's no way to know when it's going to happen... Thanks in advance!

98 JeepJeep

IAC=Idle air controll that wont effect your part throttle/WOT driving. Id swap another one from the JY...replace that O2 sensor reset the ECM and go from there.

BlueXJ

I also think the IAC could be getting a weak or low voltage signal(+ or - 5v) or transmitting one. Did you clean the connection on the IAC? If you did I would change the IAC and the O2 sensor as indicated by the CEL and see if that doesn't improve your idle.

EdW

Yes, and that potential signal issue coupled with the fact that I can't see any of the other sensors on the SCAN tool makes me wonder if the ECU is bad or some other issue doesn't exist. That's in part why I'm a bit hesitant to replace any parts at this point. Keep in mind that the idle is fine over 90 percent of the time, engine runs strong and normal, it's just when it goes bad, it's pretty unsafe. And the only thing I know for sure is the IAC activates and limits the air in when it starts to go bad. Thought about warming the engine up and unplugging the IAC, but I don't know if that would tell me anything....

EdW

Well I figured out the scan tool problem.. There are 2 modules that you can select, one is 24 which is the default and only showed the TPS, when I selected the other 209 module, I was able to see everything.

O2 sensor waveform looks fine as does most everything else. Should also mention it's been 2 weeks since I reset the trouble codes and none came back, even though the idle problem is still happening.

Checked for loose wires, cleaned up connections, checked vac lines, to no avail. So it seems to be an intermittent electrical problem and I'm about to replace the IAC motor, though part of me feels like it's going to be a waste of money as it's just a shot in the dark. Any other suggestions would be welcomed at this point!

whowey

Have you had the throttle body off ??

This sounds VERY similar to problems I have had in the past. All the tests would have normal results and the inividual components seem to be working. But it turns out the TB was full of crap...

EdW

Nope, just cleaned out the (visible) housing around the throttle plate. Where was yours gummed up, the IAC passageway? Were your problems intermittent? This runs well 95% of the time, which makes it difficult to fix! Appreciate the input as I'm about to just buy a new IAC motor out of frustration, hoping that's the problem... but my gut tells me it may be a waste of money.

elmerfudd_22

Im having a similar problem. My girlfriend just recently bought a 96 Grand Cherokee, and I scanned it just for fun. It says that I.A.C motor is shorted. The battery wont hold its charge also, I guess because of this?I dono what to do.
any help for a newbie apprentice would be great, thanks.

jonb8

My 98 was stalling and acting funny so I unplugged the temp senser, cleaned it and plugged it back in and I haven't had any trouble since. Just something to check.

ulendo

EGR? I picked up my 89 for next to nothing due to a number of issues, one of which was intermittent idle speeds /stalling / surging. eventually traced it to the IAC try to accomodate surges from a malfunctioning egr system...not sure on the '98, but on an '89 theres nothign that shows up on the scan tool.....