Thanks ! What do you use to keep the voltage at 13.8? I only have a standard battery charger and a battery tender.

 

There are many possibilities. Mine is 45A. 25A is ok for the x350s. Newer ones should have the 50A unit.
This is what I use:

 

Thank you! We're gonna give it a try again and hopefully we are able to flash the ECU. Hopefully keeping the battery at 13.8V+ won't kick us out of the reflash process.

If anyone else has gotten kicked out of the reflash and knows why Id love to know what worked for you. thx again.

 

UPDATE: I haven't had a chance to do much with this issue other than install a relatively new Denso MAF sensor I had lying around from my wrecked 04 XJ..The MAF sensor was only a few months old, so it is basically new. Unfortunately same issue. Car still runs perfect, maybe a slight surge on start up otherwise rides so nice. I did notice a "whistle" or "hiss" noise when I had the hood open. I assumed some sort of a vacuum leak, but the fuel trims were fine. I'll hunt for the cause soon and hopefully have time to reflash the ECU (if I figure out how to do ).

On the positive side installed new tires, replaced the passenger side upper control arm (was getting an annoying rattle on bumps), and alignment. The big cat is driving perfect, silky smooth.

 

I've still been procrastinating on this...Light came back on last week but since I was under there I cleaned the MAF again and also cleaned the throttle body. Light went away on next start. I doubt its fixed since mine seems to go on for a day or two then go off for a few weeks. I'm due for inspection in August so trying to get it done before then. If not I'll just take it on a day with no light (and at least 3 monitors set) and get stickers for the year and worry about this later. Doesn't seem to affect the car at all.