How do you adjust the idle on a Subaru Outback?

You don't. It's adjusted by the ECU (engine computer) and not externally adjustable. If you're having a problem with the idle speed not being correct or being unstable, there may be an issue with the idle air controller which is an ECU-controlled valve that regulates the idle speed. Over time the IAC can become sticky from internal deposits, it may need to be cleaned or replaced.

OK, thanks. I was able to manipulate the engine speed by pushing around what I thought was the idle.

You should NOT attempt to manually adjust the idle speed by turning the stop screw on the throttle arm. This is an adjustment that sets the minimum throttle plate opening but is not intended to be used to set the idle. Changing its setting will alter the IAC calibration, and the ECU will then "learn" around the adjustment. If the stop screw is adjusted too far the ECU may not be able to compensate. So, short answer- don't mess with that.

replying to stonej, BoboheadEer wrote: stonej, I work on Subarus, BMW's, Fords, Chevy's, etc and I have for 30 years. From what you are typing, you should close the hood and take it to a qualified technician. "OK, thanks. I was able to manipulate the engine speed by pushing around what I thought was the idle." Not the words the tech that fixes it wants to hear.

Correct. The idle is computer controlled. Don't "screw" with it.