It has to see two consecutive occurances to set the light. The light can go off as well, after XX driving cycles of the PCM not seeing the fault, so teh MIL going on and off is quite normal. That little crack may in fact be the source of your issue. Especially with weather/temperature changes, it may be expanding (no matter how small the crack is) and altering the system to set the code. Any litle hailine crack in the sytem along the tubes, at the connectors, or a faulty purge solenoid or actuator may be the culprit, here.

OK...one more question....let'* say it stays off a while(a long while would be good), but it comes back on, does it erase it from the PCM long enough that if I unplug it before I go for inspection....will it show anyways even though there is no CEL???

you mean Evap vent valve i thinks thats by the fuel tank

After I installed the PCM and FWI i got two codes that keep reoccuring, i first thought the P0441 (Emission evap canister incorrect purge flow) code was from not installing my evap canister back, so i reinstalled it and erased the code, but im still running both the codes, i have checked all the vacuum lines, everything is fine, no leaks and my canister is relocated, the P0171 is "running lean" , anyone know what might be causing these codes...

For the P0171, clean your battery cables, both ends, and pull the boots off from the battery end to clean off the corrosion. Do a search on that code with BillBoost37 as the author. He chased that code around for some time.