I have a 93 xjs 4.0 and was having idle and cold idle running problems. Kind of minor, the engine would run very rough at cold idle and slightlly rough when it would warm up. Once off idle, the engine seemed to run up to high rpms with no problems.
I did all the tests that I could do and got it to run better, but not as smooth as I felt it should. I finally broke down and took it into a professional. He ran all the some tests I did plus some others and found some minor problems that I didn't seem to find. He corrected those, but informed me that the MAF or the computor my be an issue, the readings at the MAF were slightly off.
To save me money, he told me to do the install myself. I bought a rebuilt MAF, but it didn't seem to make any big difference, so next was the good working used CPU. I plugged it in and it seemed to make a little better running engine. I've only let it run about 15 mins. and did not even drive it, due to snow and weather issues.
My question is, does a newly installed CPU require some break-in time to adjust all sensors etc. Should I drive it around to let the CPU adjust to all the settings.
Any information will be appreciated.
Scott

 

Not really break-in time, more like learning curve time. Yes, a swapped in CPU or ECU will require a few miles to get "settled" with the way your engine runs and how you drive.

However, even after this familiarity is accomplished, you might still have the problems you are experiencing.

It is rare for a CPU or ECU to go bad. It does happen, but rare. Most likely there is another problem that has not been addressed.

I am going to guess that you have an air leak somewhere, and are running slightly lean. I am sure a tech will chime in with suggestions to narrow down your problem.

 

You might be correct, but both myself and the repair shop attempted to find a leak and found nothing. I will see how it runs after a short drive, but that might not till next spring. I can let it run in the garage, but I put it up on jacks for the winter and really do not want to take it down.
Anyways, I will try checking for leaks again. Thanks for the advise.