I don't know what is wrong with the way my car is running, but I do know that I normally have bad mpg and a slightly irregular fuel trim on bank 1 like this:

09:20:11 Group 032: O2S control learning values (before Cat. Conv.) [B=Bank S=Sensor]
-0.8 % @idle B1S1 +/-10%
-13.3 % @part B1S1 +/-10%
-4.7 % @idle B2S1 +/-10%
-3.1 % @part B2S1 +/-10%

However, if I use a hand pump to blow some air down the fuel return line and make some bubbles in the gas tank, my fuel trim suddenly improves dramatically to this:

20:02:43 Group 032: O2S control learning values (before Cat. Conv.) [B=Bank S=Sensor]
2.3 % @idle B1S1 +/-10%
-7.0 % @part B1S1 +/-10%
-1.6 % @idle B2S1 +/-10%
4.7 % @part B2S1 +/-10%

Ok, now that doesn't make any sense to me. My fuel pressure is normal (I have a fuel pressure gauge installed also), how could this simple procedure be modifying my trim to within the norm so much?