my gas mialage is poor

try the fuel filter, pvc valve, after that fill your tank and put some injuctor cleaner in it and then run your car and the when your tank is empty change you oil even if its early.

the fuel filter replacement is a good idea. also a fuel injection cleaning at youre local gm dealer is a great idea. bypassing the converter is a bad idea because the pre and post cat o2 sensors wont read anything cuz ur bypassing the converter so you will get a check engine light on and the powertrain control module (pcm) will tell the injectors 2 get more fuel into the cylinder. so in short the o2 sensors measure the effiency of the converter so by bypassing the cat it will throw a check engine light and possibly dump more fuel which wont help you.