Originally Posted by Prometheus2508

Hmmm, sounds like a side affect of removing the catalytic converters. A problem I experienced once may be applicable...

A buddy of mine removed the cat on his 01 Impala, along with the second O2 sensor (as it is not used for A/F trims), buthis caused a severe drop in fuel economy. After some research, we discovered that this is actually a result of bad programming with the computer.

The post-cat O2 sensor is used as a reference to see if the catalytic converter is operating. Upon start up, the engine compares the readings from the two O2 sensors. Even though the cat won't be up to operating temp, there will be a difference between the sensors. If a bad signal is being read from the second O2 sensor, the engine will enter a diagnostic mode, dumping fuel into the cylinders to attempt to get any reading from the second O2 sensor. Unfortunately for my friend, the engine won't exit this state and will continue to dump fuel.

Sometimes, if you are stubborn about not putting the catalytic converter back on, you can get around this issue. My friend completely removed the second O2 sensor, but as long as it is still on your vehicle you can try it. The first O2 sensor signal will fluctuate as the engine switches back and forth from rich to lean mixture. With a functioning cat, the second O2 sensor will read a similar but extremely damped waveform of the first, which is how the computer knows the cat is working. With the cat removed, this effect can sometimes be achieved by installing one or two spark plug non-foulers between the second O2 sensor and the threads it is supposed to enter. The threading is all the same, so they should slide right in.

It should look like this:
| |
| (female threading) (non-fouler) (non fouler) (O2 sensor)
| |

This backs the business end of the O2 sensor out the the exhaust stream enough to trick the engine into reading a functioning cat signal. Doesn't always work, but non-foulers are cheap and you aren't modifying anything, so worth the risk.

Hope it helps.

EDIT: Reading over this, I am realizing I may not have been super cear about the root cause of this problem. If you want something explained, or more information about how ctalytic converters work/are monitored, let me know.