Everyone and their stupid quotes. Fuel trim adaptation is either O2 sensor or vacuum leak, but it sounds like you have a bad vacuum leak causing misfiring. Also a weak battery WILL cause misfiring. Its happened to me. Shorts can be found by removing the positive lead but leaving the negative than using a multi-meter. Put the positive side of the multimeter to the positive lead from the car not from the battery terminal, and than put the negative on the battery/terminal. If it has a reading there is a short. Fuel trims are almost ALWAYS vaccum leaks. Start with getting a can of brake cleaner, start the car, and spray around manifold. Listen for idle changes. Idle changes mean vacuum leak. And that shop who quoted you 8,000 is FU&KING YOU. I would've laughed. DEALER isn't even half that cost. Your start up rattle is 70% the side tensioner in the passenger side timing cover. Easy and cheap to replace. They've been updated because the springs weaken and compress. I paid 800 in total for all my timing parts and 300 for VANOS timing tool set. So 1100 and replaced all my timing stuff. This included coolant hoses, manifold gaskets too. That shop would've made HUUUUGE profits from you at 8k. Labor on that job is like 21-hours which comes out to no more than 2000 for the job.
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