Ok, here's the deal. I have a 98 Avenger with the stage I on it. It was running fine until about a month ago when the check engine light started coming on.

A few things I've checked:
1.Spark plugs are black as hell. Looks like I'm running super-rich. How can I take care of this? Keep in mind that my A/F is fine at WOT (about 95 at it's highest) and the EGT is at a consitent 1500 degrees.
2.I have an exhaust leak somewhere. I tightened down all the bolts on the manifold as far as they go. I didnt see a crack anywhere but I can hear the exhaust leaking. Would an exhaust leak cause a rich condition by messing with the computer somehow?

I have to get that damn check engine light off. It's so annoying.

Here is a theory:

Maybe you have a leak before the O2 sensor, which makes the O2 sensor give a false reading, therefore increasing the amount of fuel that the computer gives to the engine...???

Maybe...

Do you know what code is being tripped? Yes an exhaust leak can cause the CPU to over act and make shit rich. Check your O2 sensors. That got me. Turned everything black from the inside out.

I havent been able to get any trouble codes on it as of yet. Seems like the autoparts stores around here dont have anything I could use. I'm going to hook it up to a laptop this weekend hopefully and get some codes from it.

Could the clamp used for the hrc downpipe so that their turbo downpipe clamps to the stock downpipe leak enough to mislead the O2 sensor?

The leaking noise you are hearing is from that clampped connection. Get that thing welded! =)