Hey guys, I am a new 2000 jeep Cherokee 4.0 owner and it's definitely been a fixer upper. Since I've had it, it always started however sometimes needed twice to do so. Unless you turned the key to "on" without starting the engine twice beforehand, and then started itup, it would start perfect- I read on here that was most likely an aspect of the fuel pump assembly.

So today, we replaced the fuel pump assembly in the jeep, started it up afterwards and it took twice but figured it was cause we just put it in. But it started normal and idled normal.
After that repair, from about noon to 6pm we had other repairs going on such as changing out all the shocks, replaced the oil pump and screen, and oil pressure switch. We had the battery disconnected but I'm not sure for how long.

Once these repairs were done, we went to start up the jeep and it won't start. It tries but dies. If you give it gas, it'll start up rough and then the idle is extremely low almost feels like it's going to stall. The idle is at about the midway point from 0-1 on the RPMs, where beforehand it was at about 800-1000. The idle is low at complete stops and when you start it. Feels like it's going to stall. The temp seems to be running a few degrees higher as well, I can hear the fan going even when I'm driving whereas usually it only came on when stopped for a bit. Oil pressure seems normal.

We have no idea what is going on? Any thoughts? We don't understand why it never did this until now when we changed out some things. Any input you have would be awesome as I need to have this repaired by tomorrow. Thank you!

 

Clean your throttle body and IAC. Report back.

 

Checking everything in the morning, will get back by tomorrow afternoon! Also- does it have anything to do with the battery being disconnected for a while? Jeep is now starting up fine, fires up good but goes right down to low idle, it's about 550-600 in park and down to 250-300rpm when put into drive before giving gas. Seems to be running normal, just again about 300 rpm while idling at stop lights and such, when before today it was about 800 idle. Thanks for the input

 

Sounds like the pre cat o2 sensor. I'd ensure it's functioning properly... check for frayed wiring to the sensor as well

 

Double check battery terminals are super clean and super tight. From experience these jeeps get wacky with the slightest bit of extra resistance through the electrical system.

 

Plug into it and see if there are any stored codes.

 

disconnecting the battery caused you to loose all the learned values, the idle air control, IAC needs to relearn position as well as the throttle position sensor....good scanners allow you to perform these relearns quickly, but without them the pcm needs to learn them each time you start the engine, once relearned all is fine.

 

Jeep is now starting up great and idling normal while in park & before warmed up, once warmed up it drops down to a low idle when at stop lights and such. Think it's definitely what you're saying? i saw someone write a post about how because it wouldn't start yesterday, and we stepped on the gas to get it to start, this threw off what it thinks the normal idle is and that we need to redid connect the battery and turn it on without stepping on the gas and just let it idle for 20 minutes. Is that a thing I need to do, could I have messed it up by stepping on the gas to start it up yesterday? Or do you think it'll still work its way out to learning the correct idle position? And if so how long does that usually take

 

The PCM learns over a series of key on/key off, I can't recall the exact number at the moment, like 40 or something. So, letting it idle for 20 minutes will do nothing. Just take some short trips, it will re-learn over time.

If you still are having issues after that, try cleaning the TB and IAC as I recommended above. You can also check the TPS, I had a similar issue a while back, and found my TPS had a dead spot in it, causing a low idle at stoplights. Replaced it and all was well.

 

Great advice thank you. We are cleaning the throttle body and IAC now just preventative but I think the key on key off should do it as there were no problems before disconnecting the battery. I'll check back in a day or two and fill in on the progress