2001 Windstar runs rough and shows lean
Our driver came in today with the 2001 Windstar (234XXX miles) barely running. Give it a little throttle and it died. Restarted with a little cranking and then it ran fine for a bit. Eventually - 10-15 minutes - it would start to run rough and barely run, even dieing occasionally.
Finally, it threw a code for bank one too lean. I hooked up my snap-on scanner and watched the O2 sensors and when it was running fine, everything looked fine. The first sensors were going up and down as expected and the second sensors were OK. I just changed one second O2 sensor with a NAPA one (not my choice, the boss ordered it after I told him to spend the extra $20 for the Ford one) and that one had a much wider swing that the other second sensor.
I am concerned that could be causing troubles, but I would expect it to be a consistent problem, not one that comes and goes.
I have also replaced the intake manifold gaskets last year, but I would expect a vacuum leak to be consistent as well.
I could look at the trims, but I don't know what to expect and even though I have done lots of reading about it, I still don't understand it or know what to expect for readings.
At one time, I thought that this was vapor-locking somehow since it was rather hot today, but I don't think that is possible, is it? If I got a fuel pressure gauge, what should I expect for a reading there?
As always, any help is appreciated.