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Hello there, I just joined. Last Saturday, I bought a 2003 Impala 3.8L for my girlfriend because her last beater got wrote off by a dude that ran over it in a parking lot with his huge pickup. She's unemployed so I went to an auction and bought it for her for cheap. It has 145,000 miles on it, but is in very good condition, very little rust and appears to be well maintained.

I'll try to explain the issue as completely as I can with the hopes that someone has experienced the same and can confirm exactly what's wrong so I'm not chasing and replacing a hundred things.

The vehicle starts and idles fine. Upon startup there's a loud sucking noise coming from the airbox before the filter, which I know is normal because of the IAC during cold startup.

If I start driving right away, it accelerates through the gears and shifts smooth with what I would call normal acceleration. Feels like it should. After a couple of blocks, or if I let the engine warm up on it's own before driving, the acceleration becomes very poor. There is a more hollow muffled sounding sucking noise (not even sure it's air, but that's the best way I can describe it...) during the poor acceleration. It is the same through all the gears, shifting seems to be normal still, however.

If I press the accelerator maybe 2/3 of the way down so that it goes into the first passing gear, the engine revs higher, the hollow sound goes away and the engine sounds more like it should. It accelerates much better, but I'm not sure it's as good as it should be.

If I floor it so it goes to second passing gear, engine RPM's go higher, but acceleration isn't as much as it should be and there is a loud rattling like marbles in a tin can. This rattling is worse as the engine gets warmer.

After doing lots of reading, I'm suspecting it's the cat, but I'm not sure why it would be ok for the first couple blocks before it warms up. Secondary suspect is plugs, wires and coils. Wires and coils appear original and I'll likely replace them anyway. Pulled the front three plugs and they all look great, I was quite shocked actually. Will probably replace them anyways.

In Park, the engine revs up nice and sounds normal. This is also confusing to me since if the cat was plugged, shouldn't the engine have the same noise and performance problems as when it's under load? Because it revs up fine and sounds good, I'm ruling out a fuel delivery problem.

So, I'm kind of confused. If it's a transmission problem, why does it shift and run so good for the first few blocks? If it's a cat problem, why isn't a problem right at startup? The tin can rattling makes me strongly suspect the cat though. Could be coils and/or wires because they tend to do less well when hot and under load, but the engine does sound fantastic in park.

Another question... will pulling the front O2 sensor allow enough air through the port to allow me to diagnose the cat? I don't really feel like disconnecting the whole pipe or manifold before the cat.

Any help would be truly appreciated. Thanks in advance.

Scan shows no error codes... I plan to clean the MAF sensor, but looking at the readings on my OBD tool, the numbers don't seem out of line.
 

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Update: For anyone that cares, I found out it was, in fact, the cat. I pulled the O2 sensor (should have just done this from the start) and it ran way better. It wasn't 100% performance, but went from un-driveable to very decent performance and it shifted up smooth through the gears. Replacing the cat today.
 
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