Hello,fellow Subie enthusiast.I've encountered a problem with a 98 Subaru Outback 2.5 DOHC AT.I got the car for a very cheap price because the po tried to do a timing belt change,and couldn't get it in time.Well,I took the covers off and got all of the marks lined up,and tried to start it.It turned over,but wouldn't start.I discovered if I sprayed ether in it would start,and run as long as I kept spraying it.So,I changed the fuel pump,and filter.It will now start,and run as long as you hold the gas pedal down.If you let off the pedal the rooms drop,and it dies.It threw a po183 for fuel temp sensor after I changed the pump.Then it stopped throwing that code,and started throwing the code for a knock sensor.(I can't remember the number for it right now)so I changed the knock sensor,and now it's throwing the po183 again,and still won't idle.Im pretty certain that the timing is right,but now I am starting to doubt myself a little.It also makes a whistling sound when you let off the throttle,which I thought was a vacuum leak,but I can't seem to find any leaks.Sorry about the long post,but any help would be appreciated.Thanks ahead of time for any suggestions.

Suggest you look long and hard for a vacuum leak. That whistle sound may be telling you there is a vac leak. A vac leak will indeed cause a poor idle, like what you are experiencing.

Check the intake manifold gaskets.

check the large hose from the IACV to the underside of the intake.

 

check with a vacuum gauge

 

sometimes, spraying starter fluid, brake cleaner - even propane around the intake manifold and vacuum hoses could help find the vacuum leak - the idle will smooth-out.