Engine will not accelerate past 1200 rpms when in drive and idle is rough in park

SOURCE: rough idle / engine surges until power is applied

Sounds like a possible vaccume hose leak?

Jerry Koski

• 443 Answers
• Posted on Apr 12, 2009

SOURCE: rough idle, backfire on acceleration

check everything you can under the hood your air intake system all of it especially make sure all clamps are tight .. and make sure all vacuum hoses are good and not cracking or broken or just loose . you might run a little fuel cleaner through a tank of gas too cleaning the engine fuel system up some. just a few ideas.. Jerry

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

SOURCE: rough idle. rpms drop. misfire.

try your coil packs they go over your sparks or do plugs first and see what happens do affordable first

SOURCE: 2003 F-150 p/u...rough idle @ stop and then

idle air control is bad