1998 Subaru Legacy Outback--Rough Idle

there are two ways your car controls its idle
the throttle position sensor and the idle air control.
if the idle is rough or wanders get the idle air control checked and cleaned up. its the most common poor idle fix

get the car scanned to find the code. many autoparts stores will scan the car for you or you can buy the bluetooth tool that connects to a phone or tablet that has bluetooth. i suspect your spark plugs might be the issue.

u should definitely get ur engines computer system scanned to avoid misdiagnosing ur isssue. once u retrieve ur eng codes u can then begin to decide which one of ur 25 sensors are acting up, or where to start lookin an testing for ur fault..... good luck

Knock sensor has gone bad most likely. I had the same problem with the same car and replaced the knock sensor. Fixed the problem. Very common with Subarus. It's an inexpensive part but it requires patience and small surgeon's hands to get to it and replace it. Lots of how-to articles on line.

its all in the name

fuel pump or fuel filter or even fuel modulator if so equiped

I suggest you get one of those manuals people get nowadays to do
it yourself.. My nephew use it to repair my car and it's totally repaired now. Here's the website he got it from. www.reliable-store.com

Could be the Idle Control Valve?

We bought our H6 used under warranty and had the problem the day it was delivered. Brought it in to the dealer and the diagnosed it as the throttle position sensor. It is an electronic component that works in conjuction with the fuel delivery system and it is faulty, it cannot tell what position the gas pedal is in, and therefore is randomly picking power settings.
We had it replaced, and never a problem since.
There are other possibilites to your problem, and this was our solution.
Hope that helps.

I'd suspect a vacuum leak in the inlet manifold, check to see if any vacuum lines might have fallen off. This can cause a lean misfire at idle, but often it's not enough of a leak to make it lean enough to misfire at higher rpm. If a leak is not obvious, check if it is a leak by buying a can of engine start (or even CRC will do) and spray it all around the inlet manifold. If the idle smooths out while you're spraying then then goes rough again, it is a leak. By spraying in specific areas you can narrow down the area of the manifold or gasket that is leaking.

Hope this helps.