99 4.0 4wd auto.when warmed up, i get an erratic idle after about 10 minutes of sitting. it sits eand goes from 200 to 750rpms, like someone is gassing it and the letting it go. it hasnt died doing it at all, it catches itself at 200rpm and goes back up. when driving after this happens, unless full throttle is applied, it hesitates erratically as well. what could this be?i have cleaned the throttle body, idle sensor/plunger. i also have a nss that is bad(thats what my check engine code reads). could that be my issue?

 

Is the Check Engine light on?

Bad o2 sensor can throw the air/fuel ratio off and create what you have described. Check for loose, burnt, chaffed wires and a bad fuse in the PDC (black box near battery).

The EC will rely on TPS and MAP sensor at full throttle removing the o2 from the equation. You could also unplug the front 02 and see if it runs better. If it does, you found the problem.

 

i will try it, and look for fuses, thank you

 

I would also start with the O2 and just to make myself feel better i would do a compression test to see what pressures you get. Mine was doing the same thing as yours but i had a CE light. Didn't turn out good. Definitely not saying yours wont turn out to be an easy fix

 

I HAVE SAME PROBLEM... here is what i have done so far:

>new cap/rotor, plugs/wires
>new TPS
>blew out gas lines
>cleaned pump and sock
>checked injectors and TB

the next thing is getting my new IAC valve in. im currently trying to find a tracking # for it. if that doesnt do it, im going for a new coil.. and if that doesnt do it, then i gotta drop like 200 on a new pump assembly, i think.. its been very frustrating.. i think that turning it on and off like 2 or 3 times when its acting up seems to clear it up.. i think..

 

VT, look into the o2 before you throw more parts at it. Check the fuel pressure and perform a compression/leak down test.

 

yup cant stress that compression test enough

 

i think the pump was tested, and compression is good... in park, warmed up (or cold) if it is acting up, you can push the throttle to a certain point, and every time once you push it far enough, the idle will jump right to where it is supposed to be. there only seem to be symptoms at lower idle, and i just replaced my TPS, so idk... did i get a good deal, or ripped off on this IAC valve?

 

a link would help
http://www.autopartswarehouse.com/sk...%26+Drivetrain

 

It was a damn fuse, how that one fuse affected it I'm not too sure, but it idles perfectly at 750 now. I'm stunned, thanks for the help.

 

What fuse?

 

It was a 15 amp fuse in the PDC as suggested above. It was the only one blown, R/R..... And it worked. Out of the tiny fuses, it was the top row, second from the left.

 

Good price on the IAC!

Your problem sounds like what Im working on now. When coming to a stop the idle will dip under 750 but a quick blip on the throttle will bring it back up?

Im guessing but it probably was the o2 heater circuit fuse in the PDC. 15 amp fuse, IIRC.

 

i was late

 

I wish I could tell u which one it was, but after an hour of searching for fuse location layout on a 99 Cherokee for an hour, I gave up. Im amazed it was something as minute as a fuse though.