1999 Ford Taurus won't start, codes 171 174

1999 taurus se, 3.0 ohc; 89000 miles

originally had codes pointing to MAF, rough start/idle (and one no start), and sluggish/studdered accel. Cleaned MAF, changed plugs/wires, and all seemed well.

changed pcv valve, and shortly things went goofy, with regression to original symptoms, and the appearance of codes 171 and 174. Car wouldn't start; batery was needed anyway; changed it out, vehicle started w/ a little effort. Starting became a 50-50 thing over a couple days, and now it won't start at all. I replaced fuel filter (found it was original still). Checked air filter assembly; no leaks there I can see visually or by sound. Don't hear/see any leaks elsewhere, but don't know a better way to test other than visual. Tapped on iac; no luck there (if I took it off, what would opened/closed look like?). Checked plugs; they all look same, with a little white on them, indicating running lean (which goes with the 171/174 codes).

OK, so what ideas do I pursue next, and in what order would you suggest?

Thanks All!

Have fun!