High Idle only on cold start up and Heater is on.

SOURCE: 2001 Forester, Rough idle and engine loping driving at low rpm

I had a similar problem on my 2002 Impreza RS 2.5 (same engine and engine control system). In my case it was the O2 sensor on the intake manifold. This happened at 34,xxx mi. so it was covered by the warranty. Replacing the part is very easy. (two bolts and a wiring harness). Obviously I would start simple and cheap and work your way to expensive and difficult.
From my understanding its all caused from the engine missfiring. If its been a while or never, I'd change the spark plugs first. (stick with the Subaru Parts plugs, I went after market and the dealer claimed the plugs were not the correct "temperature" and had to pay for the new plugs and install although they did it without my permition. Thats a whole nother story).
Could also be the manifold pressure sensor.
Either 02 or manifold pressure sensor may cause intermitent check engine light.
Where I live Brookline,MA , Autozone will run a free diagnostic on your car by hooking up to your ECU and will give you the trouble codes and possible remidies. Of course there are no guarentee's but it will point you in the right direction.
Good Luck

Mike Butler

• 6674 Answers
• Posted on Aug 23, 2009

SOURCE: 2004 subaru forester, 99,000 miles. Check engine

Drop in to an Autozone, or Pep Boys, or similar big parts store & they will usually scan for codes free. This will hopefully eliminate all the guess work, or at least point everyone in right direction for better diagnosis. If not obvious from codes what the problem is, get back to us and we'll help you out.

Testimonial: "Thanks for the great suggestion . I'll have Autozone read the codes on Monday. Thanks again!"

SOURCE: subaru impreza rs 2.5 insanely high idle

check you dont have a split in the corrugated pipe from the maf to the swirl chamber (the big black plastic tank attached to the inlet manifold) or a leak into the intake somewhere, doesnt have to be a small vac tube.

Also you may have a sticking TB butterfly as my learned friend suggested.

SOURCE: My Subaru legacy grandwagon has started cutting

You could have a number of problems. First do a complete tune up, proper timing, new plugs etc.

Turn the key and do not give it gas. If it starts slowly give it gas and let me know what it does. I suspect it will cut out. In that case, you might have bad injectors. Let me know if I can be of further help

tom

SOURCE: High idle

Your car may not have recieved a recall on 02/201998 on the vehicle speed control recall id from NHTSA98v035000. the throttle shaft ball bearings on the sensor side of the throttle body assembly was not insalled in certain throttle bodies. Operation of a vehicle with a missing throttle shaft bearing can eventually lead to an incomplete return of the throttle valve resulting in a high idling condition. 61115 vehicles were affected. Take vin number to dealer see if your vehicle has been inspected for this missing bearing.