88 v-6 2.8 5spd. I'm running a test pipe in place of my damaged cat converter. No emissions laws here (yet). I'm getting about 14 mpg city which I think is pretty bad for a v6. My LT1 gets about the same or better. My air pump is still working.

I'm wondering if the O2 is reading leaner than it really is because the air pump is still pumping or is it to free flowing for the O2 to read correctly, and possibly adding unecessary fuel to the mix?

Can I relocate the O2 up higher but I'll be in only one bank and get a better reading?

 

You really don't need an air pump when the cat is removed, as its puprpose is to make the cat light up faster. And as you have mentioned, it could make the O2 sensor see a leaner mixture due to the extra air introdiced in the exhaust.
If the pump is electric, it should shut down when the engine goes into closed loop.
Is the check engine light on?
Regardless, disconnect the pump, and replace the O2 sesnsor just in case it got fouled up when the cat went bad.
14 MPG in the city is not bad, as the MPG of the car in city driving is determined much more from car weight than engine size

 

Thanks Pete. The only code I got was for EGR which I think was 32 or 34 because I had plugged with a block off plate to the intake because the valve was sticking slightly open causing a rough idle. O2 is new.

The other reason I was thinking of relocating the O2 is that it's factory location is right after the Y in the pipe and it's really close to the ground, Which is probably why the cat got damaged. Also I thought maybe it might be better being up higher and closer to the exh manifold will allow the non-heated O2 sniff alittle better. But I don't know if reading only one bank is a good idea?

 

You probably get away reading just one bank a slong as the other bank is working OK. Being that you have a check engine light lit, it is possible that the engine is not going into closed loop, and that would definetely give bad fuel mileage.