I was wheeling yesterday and i noticed that my 02 sensor was snapped off. its a 92 obd1. I'm not getting any cel tho (seems really weird to me).

I also have a cracked exhaust manifold. I really don't want to pony up the money to replace my manifold right now. would it be ok to just replace the o2 sensor? if I do is that going to help my mpg or is a properly functioning o2 going to read the cracked manifold and make it worse.

 

You will likely notice a decrease in MPG without the O2 sensor. The PCM will stay in open loop causing the problem.

Leaking exhaust manifolds will suck in additional air causing the O2 sensor to report a lean condition. THe PCM will compensate and add additional fuel. Again the decrease in MPG's.......

Hope this helps