Recently I have replaced the motor in my 98 grandprix gt. I replaced it with a 99' buick lesabre 3.8. When the former blow blew( intake gaskets go figure) it plugged the cat... So i cut it out and just straight piped it... I have heard about the 02 retarder from zzp performance... But currently I am averaging barely 20 mpg on the highway! Is the rear 02 sensor being causing this??? What are your thoughts