I was thinking it might be the timing, but it sounds like you found the problem. Here is something to others to consider if nothing above helps. I recently replaced a timing belt on a double DOHC motor. It was tough job because the timing gears were spring loaded and there are 4 of them to keep in place. The first time there must have been some slack in the belt, thus throwing off the timing slightly, which created a P0171 Lean condition. After an extra day of frustration I got the timing aligned precisely and never had any more codes.

This is nto an issue with the 3.8L. Reason #423,233,443,525,245 why I hate DOHC'* though.

Timing is never a problem on a 3800.