Engine Knocking Noise Problems of Mazda CX-7

My 2007 Cx-7 started making a loud banging noise when I started it (may 7,2012). I brought it to the dealership to have it checked out and they said it was my timing chain & actuator (vvt) and it would take $1350 to repair. I paid the money and it was fixed in a couple of days. On June 7th,2012 I started to realize that my ac was not working, but had decided to wait a little bit to have it fixed because I was still hurting financially from the last fix a month ago. Than on June 9th, 2012, I went to start my vehicle and it sounded like it was about to explode. I live in the country and the closest dealership is at least 50 miles away. So I brought it to my local repair shop and they told me my turbo is blown. The Mazda dealership wants $80 to diagnose it and $5000 to fix it (that doesn't include my towing fees and the cost of fixing the ac). I've read all of the complaints online and everyone is saying sludge build up and Mazda wants the receipts for all of their oil changes. I have all my receipts for the synthetic oil changes that I have had done to the vehicle on time (not to mention only filling the car with premium gas). Unfortunately, my car is up to 100,000 miles and they only extended the warranty to what. . . 70k? so basically I have a car that I paid $16k for and is completely useless to me only after 100k miles. From what I've researched vvt's are not supposed to go out until what 250k-300k miles? and it seems a bit ridiculous for the turbo to go out after 100k miles. . . .

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I bought my 2007 Mazda Cx-7 on November 2, 2008 from tustin Mazda. From day one I have experienced an intermittent metal knocking noise while accelerating. It sounds like 3 or 4 clicks and I experience a slight lose of power. I have taken the vehicle back to the dealer more than 4 times. Sometimes they have it for days; the last time for two weeks. The first few time they said they could not replicate the problem. About a month ago the dealer said they fixed it. They also said my oil was sludgy and suggested a oil flush. I did not op for the oil flush because my regular mechanic said they are over-rated and a few oil changes should clear it up and the oil the dealer used may be prone to slugging up. The noise returned almost the day I got the car back. I returned two to three weeks later and told the dealer service people the noise was back. They kept the vehicle for two weeks and charged me $252. 00 to run tests. I was told I would not have to pay if the work was covered under warranty. They opened the engine up and said the engine was sludgy. They told me Mazda of America would not cover the work and I might need major work. I am supposedly losing compression on two of the four cylinders. Because I cannot prove I changed my oil and do have receipts beyond the oil changes I had at the dealer, they say it is not covered under warranty. The vehicle only has 36,000. I think it had about 15,000 miles when I bought it when I bought it. The dealer is trying to tell me they fixed the problem I had been bringing it in for and the noise I hear now is new. It is not, it's the same! in fact I heard it again the day after I brought it home. I have done research on the web and it appears Mazda uses the same excuse for a lot of similar issues and do not cover the work under warranty. They also refused to fix a recall issue I had in regards to a letter they sent me. They said I did not need the work even though the issues I have described are also described in the letter.