1996 Impreza Outback, 2.2: 5 speed has developed an intermittent rough idle. I noticed it when I came back from a highway drive. It only happens at idle, it takes off fine and runs fine, no check engine codes either. I replaced the fuel filter, put some seafoam in the vac line as well as a gas tank because I am thinking it may be a dirty air idle controller or a dirty injector. What confuses me is that it doesn't always run rough. Sometimes it runs perfectly smooth at idle, warm or cold, then 5 minutes later it will start to run a little rough. Not a misfire but a sort of out of balance engine feeling in my seat if that makes sense.

I see nothing about starting off with a tune-up. Wires, plugs, PCV valve.

 

How many miles?

 

I am going to assume no CEL, so at this age yes it can be a tired

idle air motor.

Looks like the previous owner just did a tune up, wires look new. I'll pull a plug tomorrow to see. It has 185,000 miles, didn't check the pcv vavle

On subaru engines it is amazing what the PCV valve can do when it is clogged.

Idle was smoother this morning so maybe the seafoam helped, it still feels a little rough but not as sporatic. Didn't realize the pcv was only 6$ lol, ill change it and report back

So after my last post the rough idle came back, but it looks like the PCV valve replacement has fixed it. That or it was a combo of the PCV, new fuel fitlter, and Seafoam that fixed it hah