回复：my car is not under warranty(2nd owner)

The diagnoses will cost some, depend on the problem. For example, it costs only 30 minutes labor to do a engine oil pressure test, but if they found out it is low, well,several hours other tests follows. The worst thing is that when they found out you need overhual your engine, typically $1500 range, or just change an engine (used $1500 or new $3000 parts plus labor), what is your call then? So my recommendation is that ask the shop to give you a written estimation on initial diagnostic cost. Then decide after you get result, whether go ahead or stop there. Draw a line that what is the maximum cost you can have that after that expanse, you can still walk away from the car. I hope there is nothing wrong in your engine, and might just need to change timing chain...