We recently replaced the blown 4.3 in my wifes blazer with a low milage JY 4.3 and on startup we had lots of misfire codes. Changed the distrubutor and they went away. Drove it around for a while and got P0171 and P0174 Bank 1 and 2 lean codes. We cleaned the MAf, the code came back. Finally it went away for about 1 1/2 weeks. The other day it came back on, but I noticed something. It happened when I started the blazer with the air conditioning turned on. Took it to my son at work and we hooked up the snap on reader. The air conditioner adds 4% to load when turned on. This moves it to another cell in the fuel map. Could those cells have inadequate fueling stored for that load? Everything else looks pretty normal, but the O2 sensors might be a little lazy. I should mention that the truck sat for 9 months with the battery disconnected between motors. If I leave the air off, we don't seem to get the code. And the fuel pressure is dead on at 55 psi.