Idle up and down

[jbetz812]

I have a 97 TJ, auto, 4.0, air. I just got it back together after my daughter hit a tree. I had the battery out for 2 months while i worked on the frame/sheet metal. I put the battery back in and it fired up after a couple of cranks. The idle was good until it reached operating temp, then the idle when up and down and started running rough. It will do this pattern after sitting a couple of hours. It ran great before the tree and never had any issues. Any advice on where to start?

 

[jbetz812]

 

[808CJ7]

Sounds like a vacuum leak, could it be you missed something when you put it back together?

 

[69jeepcj]

x2 on vac leak somewhere.

 

[superj]

Google "hunting idle"

It could be a few things

 

[jbetz812]

Would a vacuum leak allow it to start and run smoothly for the first minutes and then start running rough? I guess once the temp reaches a certain degree maybe it engages a new vacuum line/system and that is where the leak is?

 

[superj]

at a certain temp, the ecu reads off the o2 sensor.

 

[808CJ7]

Would a vacuum leak allow it to start and run smoothly for the first minutes and then start running rough? I guess once the temp reaches a certain degree maybe it engages a new vacuum line/system and that is where the leak is?

On initial start up, the engine is on a high idle to warm up faster. So if there is a vacuum leak it wouldn't show up until the idle drops down to normal ranges.

 

[kirok]

I had same issue on my '93 YJ. Ended up being the O2 sensor. Changed it out. Problem solved. Could be just a coincedence after accident. Not sure. However, from what you describe, that's what it sounds like.

 

[jbetz812]

I drove it a couple more times just on short trips(5-10 miles) and the idle has gone back to normal. It starts up and maintains a smooth idle. I am taking it to get the front in aligned and then upgrade the front bumper. This forum has been great to me along with the guys at Midwest Jeeps.