So the problems with my car have been: Running way to rich (both self-evident due to 15-18MPG and odor), awful crackling sound coming from Y-pipe, and weird little engine "spasms" every once in a while where I'm guessing faulty 02 sensor is dumping way to much gas in, and black smoke comes out of the muffler. This only lasts for about 10-15 seconds when it happens, and it's only happened twice.

Anyways:

Warpspeed y-pipe (original y-pipe is source of the noise...It's probably rusted as sheeit.)
Should I also purchase a new 02 sensor...maybe even a new cat? I'm pretty sure the 02 sensor is shot, but I was wondering if I took it out and "cleaned it off" or something, if maybe that was the only problem.

Suggestions?

 

If you havn't changed the O2 yet, u probubly should now. Thats what messed up my gas milege. The cat may be clogged to, mine was. Just be careful about removing ur exhaust, not to mess up the studs on the manifold.

 

yup those studs break easily. one of them is broken on my exhaust manifold and now i loose a lil compression. I don't have 600+ dollars to pay to replace it so i'll probably leave it how it is.

 

the crackling sound you describe is the exhaust manifold studs. a common problem with our cars. search through this forum a little and you'll find everything you need to know about fixing this issue and hints to make the repair easier.
cleaning the o2 sensor with some sensor safe throttle body or brake cleaner may help. it's definately worth a shot because an o2 sensor from nissan is probably about $90.
both these issues will add to loss in mpg.

 

The first thing I would check is the water temp sensor. I can almost bet that that's the reason the car is running rich...Let us know.