Carmax Maxcare Extended Warranty

I have only had my car for 1 year and it started knocking. Had car towed to Carmax and they sent to dealer for repair. Before they can diagnois the problem and determine if covered under warranty I must sign an authorization for an expensive engine tear down that I will be responsible for if claim is denied. Thinking my car was being repaired I called Carmax to inquire on the status and my service Representative did not know anything and told me I could call the repair place that had the car. Is that great customer service or what? After 3 weeks of calling Carmax and getting the run around,and still no approval for a rental I called the repair place and found out what Carmax could not or would not tell me. Before they could touch the car I needed to sign an authorization to tear down the engine and I could be responsible for the expense. How do I know that Carmax didn’t sell me a car that already had engine problems? They claim to do certified quality inspections but should we believe them? Is buying the extended warranty worth this hassle? Regina Ruther Glen, VirginiaU.S.A.