I have a 1991 legacy wagon 4x4 when I start it when it is cold it starts up perfect idling well and everything. Once it is warm if I drive somewhere and then start it back up it bogs down all the way to almost dying then bounces up then goes down and does that a couple of times before idiling around 500 which I still think is too low. very annoying. giving it gas doesnt seem to affect it either. once you release the pedal it still does the same thing. any ideas what it would be?

Most idle issues can be traced to the IAC valve.

The ECU controls this valve and when it gets gummed up

you can get what you got.

 

I did a search in the New Gen section and turned

this link for you (if you try a search put idle air control in quotes)

Hope this helps.

Cleaning the Idle Air Control Valve

Coolant Temp sensor might becausing that weird idle.

Yup, sounds like a dirty IAC valve to me. Just unbolt it and clean up the moving bits with brake cleaner and a Q-tip or something similar.

I've got a 06 Impreza that's doing nearly exactly the same thing, haven't taken it in yet to get it fixed under warranty. OK when cold, but start up hot (usually worst after about 10 minutes standing while hot) and it hunts around at a very low idle speed, if you bring the revs up with throttle usually it will stall when you take your foot off, very annoying!! Seems to sort itself out though after you drive off, so problem only shows up just after starting while hot.

Can you pull the codes from the computer? A stored trouble code, if there are any, could point you in the right direction.

 

Removing and cleaning the IACV is always a good idea in my mind.

 

And, while I don't believe in throwing parts at a problem, there's a good chance the Engine Coolant Temperature Sensor needs to be replaced--it's less than $30 if I remember. A bad temp sensor can really mess with the ECUs mind, causing some very poor running.

 

Good luck!

I've got a 06 Impreza that's doing nearly exactly the same thing, haven't taken it in yet to get it fixed under warranty. OK when cold, but start up hot (usually worst after about 10 minutes standing while hot) and it hunts around at a very low idle speed, if you bring the revs up with throttle usually it will stall when you take your foot off, very annoying!! Seems to sort itself out though after you drive off, so problem only shows up just after starting while hot.

 

I hate it when i hear problems with BRAND NEW subaru's, seems so frequent now a days. Just let the dealer mess with it.

I had a similar issue with my 95 impreza, it would idle extreemly low and feel as though it was going to stall, and jerk and jolt. After a array of different attempts at fixing it turns out a new set of spark plug wires took care of all