My Cavalier is a Z24 with the original 3

Check the fuel pressure for leak down; it could be fuel pressure too high or a leaking injector o-rings causing a high fuel rate

I pulled the cat, it was fried. The guts were like a rock flopping around inside the housing. Replaced the cat, muffler, and tailpipe. The engine still runs horribly. Like it is running on 3 cylinders. Rotten egg smell. Fuel pressure 40 psi at startup, 35 psi while running. No codes, no check engine light.

No codes Hmmm. Well if your fuel pressure STAY at 35 after the engine is off and you have no codes check the EGR to see if its sticking open, make sure your ign modules are grounded, and do a compression test. It seem like you should be getting some codes if your fuel/air mixture is that rich, so if you have a scanner try to see what your o2 sensors are doing.

O.K. Guys, I spent a few days with it and found that it had a few problems coming together at the same time. First, the new cat, muffler, and tailpipe fixed the hot exhaust problem. Like I said before, the cat was toast, that is why it was glowing red. And the tailpipe had a hole in the top side directing hot exhaust at the left rear corner, that is why the taillight melted. Probably running rich on top of it didn't help. I found a couple of weak plug wires contributing to the engine running issue. But after reading your replies, along with articles on 2carpros website, I bought an IR Thermometer. Aiming the thermometer at each exhaust port and taking a reading, I found that the #1 and #4 cylinders were running about 60 degrees cooler than the rest. Since the #1 and #4 cylinders are connected to the same coil, I swapped out that coil. The car runs like new now. I am planning to do a complete tune up soon with all new coils, plugs, and wires. Big thank you to 2carpros, saturntech9 and jay4cars1 for your help diagnosing my car and getting me back on the road!