im thinking i have that same condition.With help reving the engine,exhaust pressure appears strong on the hand behind each tail pipe.maybe it should be more but im just wrong on the pressure i feel.also on acc,right at 1st. to 2nd.shift point i hear a clatter from under the hood.

 

One of my buddies had always wanted to try the seafoam treatment through the manifold of a gas engine (see YouTube videos). I volunteered my 2003 SS for it. I knew enough Seafoam could plug my cat pretty well, but I risked it anyways. a little under 2 cans went in my intake that day!

Fast forward 2 weeks, I get a misfire light. Another week and my exhaust is rattling like a b****. I lost significant power and MPG. I took my plugged one out temporarily (2 weeks). Then, my mechanic called and said he'd a new Magnaflow sitting around that would fit. Charged me $100 for the whole deal, and it sounds and runs much better now!

 

I have a 2000 Monte Carlo SS 3.8 and I have a cat from eastern Manufacturing but the in/out pipes are 2 1/4" Is that right or do I need a cat with 2.1/2"..

 

Well stock is 2.25. I had my mechanic do some extensive exhaust modification and switched to 2.5 that day