Engine stumbling when cold

[hgeranium]

I just started a new job with crazy hours, so I haven’t been able to go out and tinker on it. I thought I would ask you guys so I have a starting point when I do have the time this weekend. When the truck isn’t warmed up, and I am stopped, when I try to accelerate it will initially stumble. It seems like the power cuts out completely for a second, and then comes back. A lot of times the truck ends up dying but it always turns back on with no issues. Once it warms up the problem completely disappears.

 

My initial thought was a vacuum leak but it seems unlikely with how simplified my system is, plus I looked over it and it’s fine. Also seems unlikely considering it idles perfectly, it only stumbles upon initial acceleration from a stop. 
 

I have a pretty new idle air control valve but I suppose with the quality of parts these days anything is possible. I’ll pull it off right now and check it out. 
 

if it were a fuel issue, I would expect it to be more prevalent and not go away when it warms up, plus I have a brand new fuel pump and filter. I can check my fuel pressure this weekend just to be sure. 
 

In my mind it has to be some sort of sensor, but I’m not sure. Has anyone experienced something similar?

[AnotherOldJeepGuy]

I recently had issues that I apparently fixed, but I had to give it about 25 miles running bad for the computer to realized it was fixed.  If this started with the new IAC, or you just replaced the IAC attempting to fix it, there's a change you just need to school the ECU.  The difference for me was night/day before and after the 25 miles of training.

 

Also a common suggestion here is to make sure all your exhaust/intake bolts are tight, because they do tent to work loose.  I know for a fact I improved things by finding a few a little loose and snugging them up.

[hgeranium]

19 minutes ago, AnotherOldJeepGuy said:

I recently had issues that I apparently fixed, but I had to give it about 25 miles running bad for the computer to realized it was fixed.  If this started with the new IAC, or you just replaced the IAC attempting to fix it, there's a change you just need to school the ECU.  The difference for me was night/day before and after the 25 miles of training.

 

Also a common suggestion here is to make sure all your exhaust/intake bolts are tight, because they do tent to work loose.  I know for a fact I improved things by finding a few a little loose and snugging them up.

I tightened the bolts a few months ago but I suppose it’s possible that they loosened up again.
 

This started out of nowhere. My IACV has been working great for months. It’s also possible that it crapped out on me as it’s definitely done that in the past. 
 

what do you mean by schooling the ECU?

[AnotherOldJeepGuy]

As I understand it the ECU makes tweaks based on real time sensor input as well as historical inputs.  I've been told here it can take as much as 40 startups for it to optimize.  So if the IAC was a recent replacement to fix the problem, it could have actually fixed it but you might not be able to tell right away.  This over time adjustment was what schooling the ECU was meaning.  But since there is nothing you have just changed, that theory falls short.

 

If you are really bored, you can read my recent conical that ended with this education being at the end and the step that finally got the issue from still failing to not failing, but bring some snacks, it's a long read 

[hgeranium]

1 hour ago, AnotherOldJeepGuy said:

As I understand it the ECU makes tweaks based on real time sensor input as well as historical inputs.  I've been told here it can take as much as 40 startups for it to optimize.  So if the IAC was a recent replacement to fix the problem, it could have actually fixed it but you might not be able to tell right away.  This over time adjustment was what schooling the ECU was meaning.  But since there is nothing you have just changed, that theory falls short.

 

If you are really bored, you can read my recent conical that ended with this education being at the end and the step that finally got the issue from still failing to not failing, but bring some snacks, it's a long read 

Lol I’ve been a silent observer of that thread all the way through 

[pizzaman09]

My parents used to have that issue in their 97 ZJ with the 4.0L.  It was primarily caused by a cracked exhaust header that the crack closed up when the exhaust heated up.  It also needed a new O2 sensor, which significantly reduced the stumbles even though we never fixed the exhaust header crack.