1990 Jeep dies when letting off throttle after up to temp

I searched and found similar problem but none that matched my identical issue. Ive also tried some fo the suggestions in those threads with no luck

90 4.0 L auto





Ive been fighting this issue for over a year, here is the story so far.



Jeep developed a rough idle when warm, then eventualy it started dying when you would let off the gas while driving, and when comming to complete stops. I learned to keep it running by putting it in N and holding the throttle at approx 1200 rpms until stopped. If it dies and you restart it it idles fine, but it will not idle if you give it gas and then let off.





I thought TPS. I replaced it, and it cured nothing. I unplugged the TPS and the problem went away. But with the TPS unhooked it idles rough when cold and has a dead spot in it.



I figured that meant the new TPs was bad. So I replaced it with a differant brand. Problem stayed the same, again I unhooked the TPs and it runs fine once warm now.



I have messed with the EGR, I unplugged the vaccum hose while at idle and no changes there and I can watch it move in and out when I rev the engine.



I removed the IAC and it moves in and out when you turn the key on, I started ti without the iac in and it moved out to try to compensate for the air flow.



I left the IAC unplugged with TPs plugged in and the issue stayed the same with no other effects.



Did the same with the EGR valve.



I've tried TPs plugged in with both the IAC and EGR un plugged, problem is still there.







What am I missing?

How long has it been since you changed your O2 sensor? Since you have checked everything else, you might have a mixture issue and not a mechanical one.

I have never changed it, and from my paper work from the Original owner in the glove box, I dotn think he ever did it either.

These things are supposed to be changed regularly, around every 30K or so. They can give false readings from just wearing out, similar to a solar panel over time and not throw a code unless the heater in them does not work. I suggest you change it. Even if it doesn't fix it, your milage will probably improve enough to pay for it. I am thinking your mixture is so far off it will not support combustion at idle.

the only other thing is you might try resetting the computer if it isn't the O2 sensor.