Car stalled and engine knocking after service test drive. Background info is 2004 Grand Prix GTP w/ 75000 miles. The car is owned by my dad but used exclusively by his GF. I did have 2 days use of it, the day before going to the dealership, and the day of the dealership. Problem: The car had slow actual acceleration under heavy/full pedal use (heavy to full acceleration resulted in little actual acceleration. During the period of attempted acceleration there was also a semi rapid click that was described by someone earlier (non dealership test drive) said it sounded like spark knock. Suggested I take it somewhere to test the exhaust/cat converter which I did and they did some back flow test and said it was not clogged according to their test. Aside from the heavy acceleration problem, the car drove and sounded normal, no issues at all during normal driving. None described by the GF or noticed by myself for the 2 days I had it. Drove it a few hours both days. Took the car to a chevy dealership to get the ignition recall done and also figured I would let them diagnose it for the $32 fee. After doing the write up with the service writer I requested a tech to test drive/ride along to show first hand what I was describing. They come out with a trainee to ride along with me. Didn't really have much to say, kind of laughed said that's funny it drives normally, yet has no power during accelerate but no codes. 2:46 PM got a call from service writer saying the recall was done, and said something along the line that the power issue was an unkown internal engine problem and would need 4 hours labor @$400 to take the engine apart to accurately diagnose. Figuring that it was still fine for normal driving, I opted to decline and just pick it up and find an alternate place for someone to diagnose/look into the engine problem. Said he needed to finish writing up the paper work and would need about 30 min to finish. 4:53 (2hrs and 7 min later) I called them because I had a ride lined up that was wondering what was going on. It's all set they say. I get dropped off, get my paperwork, pay the bill, and they retrieve the car and pull it up outside and turn it off. I wait a few min to be able to talk to service adviser before leaving to go over the the notes on the paper work. I go out, get in the car and start it up and my gut drops! The engine is now knocking and shaking. Very noticeable, not good at all, can't even consider driving it anywhere. I go in to find out what the deal with that was, figure out what happened/changed while they had it. That's when the adviser tells me "yeah, one of the techs took it for a test, it stalled and had to get towed back. Of course, now that it was after 5, all the techs were gone except for the quick lube people. Can't find out what exactly happened during the test. Now I'm wondering did they wait until almost 5 to call instead of the 30 min originally stated so all the techs were gone. I did talk to the service writer and the service department manager before I got picked up, and will be going back today. Service manager actualy said some ridiculous things like "an engine doesn't just start doing this, it's been like it for a while" and perhaps someone (implying myself or the owner) added something to hide the knocking". Anyone with advice on the situation, insight to the problem with the engine. Is it just something that happens, not their problem at all and take it somewhere else or should I aim for a discount on them doing the engine disassembly and diagnoses.