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Is this the first issue you've had with the engine since new? One other thing you could try to check and see if it's a vac leak would be to spray just a little fogger (using the thin red tube attachment) at the base of each injector while the engine is running, and listen for a change in idle, which would indicate a bad seal and air leak. Also, the new plugs that you removed, do they all look the same or does one look different from the rest, indicating a problem with one cylinder?