BMW sevice engline light, po171 and 174, evaporative emissi

My service engline light keeps coming on when my fuel level falls just below half a tank.  I always get the same two codes when I check the computer PO171 and 174, too lean.  If I mess with the gas cap, take it off put it back on, the light turns off, but it always comes back on when I hit that half a tank mark.  What might be going on?  Bad gas cap, or??
Thanks!

Answer
    Ordinarily I would suspect a vacuum leak downstream from the MAF sensor.  The gas cap bit does sound a bit peculiar.  Do you notice any suction around the cap when you remove it?  If the tank vent system is malfunctioning, then a large enough vacuum might build up in the tank to prevent the pump from being able to supply full fuel pressure.  I would have the evaporative emissions system checked, including the canister.  It could be that the fuel pump is nearing the end of its life and the small obstruction caused by the clogged vent system is taxing its ability to supply full pressure.  Have them check fuel pressure after the engine has run for a while, and before taking the gas cap off.  If it's low, then a good overhaul of the evap emissions system and try again.  If it's still low, then the pump probably needs to be replaced.
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