My 88's catalytic converter was taken off and replaced with a straight pipe by a previous owner. Could this upset any of the sensors, or could the lack of backpressure put the whole system out of balance in any way? The car idles well, but fluctuates about 100-200rpm. I plan on replacing the pipe with one of Rodney's cat cons soon, but I was wondering what effect the cat had on the engine as a whole.
Thanks!

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I did the same and experienced a mild idle problem. The oxygen sensor may not be up to temp, and I know it appears longer to get to temperature. At least my Fiero does idle better with the cat on. I take it off and on all the time for inspections.

When I took off my CAT, it was really nice.

The idle, and low end torque was much stronger and steadier.

If it varies a little, I wouldn't let it worry me.

However, that could be telling you that your spark isn't strong enough.

My idle also improved greatly after I upgraded my coil.

My cat is gone and my car idles fine.

If removing the cat caused an idle problem, it's probably because something else is messed up.

My idle always fluctuates 100-200rpm. This is on a 91 Olds Calais 2.5L.