My car seems to be running crap since having the exhuast installed. Idle seems to stay at about 550rpm but the cars feels like its vibrating and if i give it the boot and get to 4000rpm it kinda loses power and when i take the foot off sometimes it will stick on 4000rpm for a second or two then drop. Wondering if the guys at the exhaust shop stuffed my o2 sensors could that cause this problem? Few days later i did a days work with them for a trial and they were shonky as hell with thier work so....
Taking your foot off would shut the throttle body and almost immediately lower the rpm, even with defective O2 sensors, unless the car is in the middle of a forced gear change. Sounds more like a throttle body issue to me. Btw 550 rpm seems a bit low, it usually sits between 590 and 610 on mine.
Plug a scan tool and check O2 sensors voltage and fuel trims, it will give you a good indication of what's going on. Concentrate on 02 sensor 2 (bank 1 and 2) because this is the only ones that would have been touch when fitting the catback, as the sensors 1 are on the cats and don't need to be taken off, unless you had new cats fitted too.
HTH
