I have a 2000 GCL with 4.0L 120k miles very clean nice Jeep. When I start it if fires right up runs smooth for about a minute then then runs very rough for about two or three minutes then runs great until it cools off again.It is not showing any codes, I have replaced the IAC. Then same thing. I have replaced the CPS, the TPS ( I not positive that what is called. It's the one on the throttle body plate closest to the valve cover and the one next to it. I have also replaced the plugs, cool rail and checked all the ground straps. Any ideas would be much appreciated.

 

Codes?
What's the fuel pressure in psi?

 

The one closest to the valve cover is the Idle Air Control (IAC) and it could be carboned up and sticky. Remove it, clean it with throttle body cleaner, and re-install it.

 

I missed putting this on my first post. It has no codes or any in memoryI don`t have a way to test the fuel pressure. , I have replaced the IAC. it should say coil rail.

 

I am suprised no one has had an issue like this.

 

Remove and clean the throttle body thoroughly. Usually you clean the IAC also but you said you replaced that.

 

I have pullet the throttle body and cleaned it twice. looked for vacuum leaks. nothing yet.

 

 

vaporizing, why would it do that when it's cold from sitting overnight? Seems like it would have to be hot.

 

if you read his initial post he mentioned it runs fine but after warmup it begins to stumble for a minute ie after heat has built up.

 

try replacibg the fuel pressure regulatorl/ filter assembly. you will need a quick connect tool. its under the jeep above the rear axle oreillys carrys them for $17 house brand. this could also be the culprit

 

I read his response, it does it about 20 seconds after cold start up then runs rough for about 2 to 3 minuets. After that it runs great

 

I'm thinking a slow responding 02 sensor. It's keeping the mixture rich until is starts functioning. The 02 sensor has to reach around 600* before it starts to send information to the PCM.

I noticed this while waiting for my newspapers to be delivered to me in the winter. I'd idle to keep warm, then shut it off. After the jeep got cold inside, I'd fire it up again to keep warm. The coolant temp was still hot, but the pipes were cold. It would idle rough until the 02 got hot again. It took me quite a while to figure that out.

 

is their a way to test the 02 sensor? And the upper or lower 02sensor?

 

You can, but it's difficult because the signal is in microvolts and you have to know it's temperature at the same time. A DRB scan tool will show you raw data.