Help Quick!! Cooling Problems

I took my 98' legacy GT Limited in for an oil change and coolant flush tonight. I picked it up after the service dep. closed, and my bill had a note saying "Needs T-Stat & Gasket. Not making good heat. Engine may overheat" and then quoted a cost to fix it. I decided to try to make it home. Well i made it but had to stop and pick up more coolant and let the engine cool down because it was overheating. It ran way too hot all the way home. I know for sure that my thermostat was fine before I took it in too get the coolant flushed. the car warmed up fine, ran right between hot and cold, and the heater worked fine. What did they do to my car???? and how can I get it fixed? I am supposed to make a 3 hour trip tomorrow afternoon and can't have it overheating. Can I fix this myself quickly and/or can I assume the dealer did something wrong? I just don't understand how a thermostat can go bad within hours.

My guess is they didn't fill it properly and there are big air pockets in your engine resulting in localized overheating. You have to fill it slowly and then you need to rev the engine.

 

Watch your coolant levels. If you need to add some after the next couple of heat cycles then you were low.

 

If this is the case, there might be head gasket problems in your future.

 

You can check your thermostat by boiling it in water and see if it opens.

 

They should have called you regardless.

That's what I had kind of thought too. So I've been filing the coolant and then run it at 2000-2500rpm for a while and let air bubbles out and then fill up the radiator again and repeat. I think I got it too the point where no air bubbles are coming out and it seems to be holding coolant level better. I'm going to check again a little bit later and I'm still going to call the dealer about it tomorrow. Is there anything else I should do/continue to do? or should it be all right now? I really appreciate the help. Thanks!!