Let me guess, upper water neck, broke at the groove for the o-ring?
That would be the one...cheap plastic POS...broke out about a 1" long spot at the groove and let the o-ring drop back into the engine. Luckily, everything kind of stuck together as we removed it, so we were able to recover the pieces and the o-ring and nothing got stuck in the coolant system.
I know that I'm dealing with an air bubble near the thermostat because I'm getting slightly "inflated" readings on the temp gauge. From everything I've read, the bleeders were put on the V6 engines, not the 2.2L or 2.4L engines. I was able to relieve some of it this afternoon by cycling the heat & the A/C. Might have to do the 0-50 thing as well...if nothing else, just for fun because I have an excuse!
Beyond the usual "let it work itself out" theory, I found this on an Alero modification site
"I found out the many-member approved method on JBO. That metal coolant line that was giving me troubles with the o-ring leaking is actually the main bleeding line. I have to make sure it's clear for flow (not bent like it is now) and run the car and squeeze the radiator inlet hose coming off the overflow tank, trying to push the fluid through the system and once coolant comes shooting out of that hose, it should be bled."
Could that possibly work?