[GM-power]

Whew that is steep, did you get a documented receipt? It's gonna have to be torn down. Since your not experienced, take it a reputable shop (alittle more $$) have them inspect the motor and find the failure. Of what they did wrong or what they didn't install etc.. get a written estimate and take it to the other shop along with pictures and tell them what's going on. If they don't want to help you out then you'll have to figure out any further action to take. Unless the paper (if they gave you any) Clearly state they are not liable for it once it leaves.