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Stumbling, stuttering idle problem


NAXJA Forum User
I have a 91 XJ that I swapped a new 4.0 into. I used the long block, intake, throttle body, and accessories from an 02 TJ. All sensors and the ECM are from a 91. All of the sensors are new.

The engine starts fine, gets 17mpg, and runs fine, most of the time. I just drove it 2500 miles across country. When operating at speeds above idle it is smooth. At idle it stumbles (still maintains ~700rpm) all the time. Here is where it gets wierd. Occasionally it will run rough for several seconds and die. It starts back up and runs fine. I thought it was the CPS, but I replaced it with no effect. All the grounds have been cleaned and checked. After the trip I pulled the spark plugs and they are burning clear, so it does not appear to be running rich.

Anyone have any ideas?