This depends on the tests that perform the engine, they must look with leak tester or a pressure meter and a vacuomether the behavior of the same, if in those tests they diagnose that the problem is valves, they possibly have to disarm a large part of the engine to repair it; Otherwise they should check what I told you earlier, I cannot say that they are doing things wrong or that they want to get money, only that several tests are needed to find the fault as such, something that I could not do; I only give you an indication of what may be happening, among others it would be good for you to shine it and indicate the fault code, which very safely will appear "Failure explosion in cylinder##" depending on that cylinder fails, I hope again is your help of something.
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