IKZ, this is the post I just did for another potential cat problem this morning. Maybe you can use it to help you out.
Suggestions from FSM are:
Checks connections at the PCM and your wiring harness for backed out terminals, broken terminal locks, damaged terminals, and bad or worn wires. This especially includes the wiring on both 02 sensors.
If everything looks OK, then hook up your scan tool and watch the screen while moving connectors and wires that are related to the O2 sensors. A change in display will indicate a problem area.
Then monitor O2 sensor activity while in Park. If the activity level of both sensors is about equal, then there is a problem. Your 2nd O2 should show much less activity.
Then, if problem is still indicated, visually inspect the cat for dents, discoloration caused by severe temps, holes, or internal rattling. Also inspect exhaust downpipe and surrounding pices for damage.
Then, if you determine you need a new cat, there should be a direct fit one for your car available. Try carsound and other performance parts places online. The universal fit one will work, but as you said, why do that if you can get a direct fit?
If you don't have a scanner available, then you obviously can't rule out the O2 sensors, but I wouldn't think that they would make any loud rattling sounds.
Truly, from your descriptions, it does sound somewhat like your cat is at fault.
Originally Posted by IKZ
Short of cutting the bast*rd off, how can I eliminate or pinpoint the CAT?
I believe that your cat is bolted on at the downpipe and at the outflange. Unless it is aftermarket already. You should be able to get a direct fit hi-flow cat online (I think, for your year).
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s