4.0 sporatic idle.

[Dwskevin]

When I come of the freeway and slow down my idle drops and raises. It drops to 500 and jumps to 1500 over and over no one can figure it out. Newly Rebuilt 4.0. 3 different distributors. Replaced the timing chain. No codes come up. 2 shops have tried to figure it out with no luck. Any ideas? Thanks

[nickyg]

Have you looked into the Idle air control valve or throttle position sensor?

[Dwskevin]

Yes, both shops checked those exact things thinking it would be an easy fix. Thanks for the reply

[4.3LXJ]

Then I would suggest pulling the battery cables and touching them together for a few seconds to reset the computer. Be patient, these ECMs are slow learners

[Dwskevin]

Great, I will give that a try.
Thanks

[gregs94]

can always spray the throttle body with cleaner to get rid of any build up, and clean the throttle blade real good, any carbon around the blade will cause it to idle funky if its not metering the air correctly, always worked on my XJ to improve idle..

[Dwskevin]

Ok. I cleared the computer so I will
Drive it this weekend and see what happens. I will also clean the throttle body.
Thanks.

[gregs94]

any results?

[Dwskevin]

It wouldn't start? I left it at work parking all weekend. I'm leaving town for a week so I'll have to get to it when I can. Thanks for checking in I appreciate it

[Dwskevin]

Update. Still having problems with the rebuilt 4.0. We got it to idol and run by modifying the distributor mounting bracket. We ground out one side of the bracket so the distributor could rotate a little more which solves the problem. No idea why that worked? What is wrong inside the motor? Yes the timing chain and gear have been replaced again.
Now smog CERT Is due and it's running with extremely high Nox. All sensors and cat have been replaced. What the heck is wrong with this motor?
I just want to get on the trail.
Even though it's not running right I still went head and finished the install of the front Clayton 3 link.
Thanks
Kevin

[4.3LXJ]

High NOX is usually due to high compression or timing too advanced. Since you have rotated your distributor more than normal, you might want to check your timing. When you put in your dizzy, rotor should be pointed at about 4 o'clock position at top dead center. If yours is not like that, then that may be part of the problem.