You don't need 500 miles to check MPG. Get the engine warmed up, find a close Interstate (no traffic lights), reset the mpg on the dash computer, then drive 25 miles - make a U turn - drive another 25 miles back to where you came from and check the avg mpg. Easy.
Longterm fuel trim: Your vacuum lines are getting old and porous. I assume you have a turbo engine, which increases the number of vacuum lines and adds a Line to just before the turbo intake that can get clogged.
Have you done a 'glove' test on the oil cap?
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Keeping it running is better than buying new
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