Jerks/bucks when cold

[Effieman]

My 98 4.0 has gotten real bad when it's cold. It will buck/jerk when under light throttle or cruise... think 'learning to drive a stick' in a 80's comedy movie. Sometimes feels like it dies when driving, but by the time I can push in the clutch, it's back to normal. Once it's warm it seems to run fine. never gotten any codes or light on the dash. When letting it idle to warm up it almost dies at times and the vacuum goes from the normal 20ish to between 10-15. A quick 'lift the hood and listen' doesn't provide any massive vacuum leaks that I hear....

Any ideas where to start/what to look at on scan data/etc appreciated.

 

[4.0blue98]

How old are the plugs? Have you cleaned the TB and the IAC valve?

 

[Effieman]

Plugs have less than 10K miles on them.
Cleaned the TB and replaced IAC recently, less than 5K miles ago.

 

[4.0blue98]

Assuming the replacement IAC is functioning properly, it does sound like a vacuum leak but not sure why it would clear up when warm. Could check to see if it's in open loop when cold and maybe check the fuel trims coldandwarm? Hopefully some of the more tech savy members will chime in...

 

[Effieman]

I cleaned the old IAC and reinstalled it. It still looks dirtier than the new one, but so far it's working....
Of course that's what happened last time, it worked...for a while.