I have a 2009 Subaru Impreza, 17.5k miles. It wasn’t driven for over three months. I changed oil, and it ran fine, but now, 4 months later, it’s running sluggishly, almost seems to drag while it’s running. When it idles, it surges instead of running evenly and quietly like it used to. I don’t know what’s wrong or what I should do.
Is your check engine light on? It probably should be.
Surging at idle sounds like a vacuum leak. Vacuum leaks are generally less noticeable when you’re off idle, but the car will be running lean. Its a little young for the vacuum stuff to get brittle & cracked or things like that, but while it sat its quite possible that rodents made a mess of somethings. Its also quite possible that there is actually some kind of intake manifold defect or something like that. Its also possible that something was bumped/dislodged at some point like during the oil change.
You said you changed the oil. Did you pull out & inspect the air filter? I’d pull the entire intake snorkel & inspect - you might have a vacuum leak. Or its possible that the intake is clogged up with something like a rodent’s nest (then the car has trouble getting enough air).