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I have a 2001 Subaru outback with what seems to be an electrical problem of some sort. This has been going on for about 6 months now with stuttering at idle and a weak acceleration. I have done the following: Changed fuel filter, air filter spark plugs and wires, Checked vacuum and that is ok. I also changed the throttle position sensor and ignition coil pack. The car is not now throwing codes, it had thrown p0302 and I changed the spark plug and wire, this caused the problems to stop for 2 weeks. The car then stalled and having idling problems throwing P0301, p0302 and p0304. Misfire in cyls 1,2 and 4. I changed the ignition coil pack again and the misfire problem went away but the rough idle and weak acelleration was still there. I then changed the idle air control solenoid and the rough idle went away but the weak acceleration is still there. How can I fix this?

Before anything else, I’d check the compression to rule out problems there. Serious compression problems might render other issues moot.

So far you’ve spent a lot of time & parts on spark & air. Other than a new filter, what about fuel?

Check the fuel pressure - make sure to do it under load. Check the operation of the fuel injectors - both electrical condition and spray pattern. It is best if you have a repair manual.