02 sensor housing and 2.5" exhaust

i just bought a 91 gsx and it has a bad case of idle surge. starts idling at around 1.5k and then when i drive the car for a while, it idles from 1.5k to 2k.

i have read a ton of info on this and at first, i thought it was the idle sensor thing that was causing it.

ever since i bought the car, i can tell that the exhaust fumes are going into the car, and i can smell it when i am driving. i popped the hood and the first thing i saw was the o2 sensor housing and the aftermarket exhaust. the problem is that the end of the exhaust, which hooks up to the 02 housing, seems to be bigger in size, plus, the whole where the nut connects the housing and exhaust, seems to be cracked(on exhaust).

what do i have to do to fix this? would i have to get a whole new downpipe in order for this to be fixed?

Dude post in the right forum, dont give admis here a hard time, moving topic around.

Check and make sure the ISC is plugged in. If it is, I would think the ISC itself is bad.

If theres a crack on the down pipe, thats what I would assume is leaking exhaust. You should get a new one, or you can weld up the one you have.