Terrible MPG and sluggish at low rpms

[jasoncitrano]

Ok, i have a 1991 GSX auto, have had the car about 2 months now, at WOT it sucks at acceleration now and my mileage is terrible. I'm getting about 110 miles to a full tank of premium gas. I replaced the plugs and wires, i've been reading around and i'm thinking it could be my o2 sensor but i wantto be sure before it buy it. Anything would help here considering i already brought the car in and the mechanic "couldn't find a problem" i'm completly stuck now, the car is very slow until it gets to about 2nd or 3rd gear then it boosts and accels fine. PLEASE HELP i have 115,000 miles and new turbo-back exhaust installed. and new BOV

 

[jasoncitrano]

Ok, figured out the problem, turns out the cast iron portion of the exhaust side of the turbo has a massive crack in the bottom and when i rev it up in 1st all the pressure leaks out, and for the bad gas mileage, the o2 is going nuts becuase i replaced the downpipe and there was no backpressure and it was causing the o2 to tell the ecu to richen the mixure... so thats the problem! :thumbdown