If it is one cylinder it could be a problem either with cylinder or with the head. Honestly the only way to find the problem is to take it apart. It may be something simple like a vaccum leak... but I am not sure that this is sypmtomatic of a loss of compression in one cylinder (not saying it can't be, just not familiar with it).
There are things you can test without taking apart the engine to see if the problem can be found.
First, leakdown test, of course you need the tester, but basically it compresses air into the cylinder then if it is losing 15-20 percent efficiency, then listen for where the air is leaking out of, if it's the valves leaking then you'll either hear it in the intake or the exhaust.
Second, dry and wet compression test, do a dry compression test on all 6 cylinders, at least twice each one, average them out see if there's a difference, then put about a teaspoon of oil down each cylinder then do the compression test again, if after you put the oil in the compression on the "bad" cylinder averages out with the rest, then you're looking at piston rings.
Both tests require some time and of course the right tools, and of course in the end if either the valves, piston rings, piston, or whatever is actually bad then you will end up doing a tear down of the engine, BUT if you can do these tests yourself CORRECTLY(and i do stress that you do diagnose your car correctly) you'll save up on diagnostic fees that either the dealership or any shop will charge you. Of course if you diagnose incorrectly and they just replace that certain part you think is bad then you're out of luck my friend.
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