Intermittent bad idle help needed

Greetings all! 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... Keep in mind that it idles and runs fine most of the time. Thanks in advance!

If you have a check engine light at the same time as the vehicle is symptomatic, focus on that. It is probably not a coincidence. Did the check engine light come on at approximately the same time as these symptoms? I'm assuming it did. So if you cleaned the IAC and where it seats carefully, I don't think what you are seeing is related to that.

I would try testing the suspect oxygen sensor at the sensor itself. Also closely inspect all wiring associated with that sensor; that is something that is often overlooked. How many miles on the engine; do you know if the O2 sensors are original?

Not the IAC.

The O2 sensor wiring may have gotten burned by contacting the exhaust manifold. Sounds like 12 volt heater voltage wire has intermitantly shorted to the O2 sensor to ECU/PCM wire. Look for damaged wire in the sensor wiring harness run to the O2 sensor.

Short could also be in the O2 sensor internally, carbon. Check it with a meter. Wiggle the wires to look for the short.

Reset the trouble codes after that last check, and even though the bad idle has happened numerous times, no more codes have been set strangely (3 week period of time). So I think the O2 sensor ckt may be OK? Any thoughts on where to go from here?

Am tempted to pull the throttle body off and give it a good cleaning, but I did on vehicle and am not sure if more can be done off? Have read numerous similar threads where members replace the IAC, TPS, etc and still have the issue. Would like to avoid going down that road, but it's tempting as I'm not sure what else to try...

Mike, Just read your thread about the TPS Sensor and the grounds near the dipstick base causing intermittent high idles on your RenX, seems worth checking into even though this is a '98 eh?

Am tempted to just get a new TPS anyway as on the SCAN tool readout, it occasionally reads 15 or so (normally reads zero at idle), but that doesn't seem to affect how the engine runs (in other words, the bad idle doesn't correspond to when the TPS readout is falsely high, it runs normally either way and something else is triggering the bad idle, strange and frustrating.....)

98s don't have the ground problems that Renix does.

I still stand by what I said, check the O2 sensor and definately check its wiring and the heater relay to the O2 sensor. It is seasy for the o2 sensor wires to get burned and intermitantly shorted on the exhaust system pipe.

IAC problems cause high and low idles, not rough idles. O2 sensor problems cause intermitent rough idles, so does a CPS on its way out, and dying primary HV wire, or bad Cap on the dizzy.....