So after having this issue myself and seeing the amount of new threads being created for it, I've decided to create a DIY walkthrough. I will try to break it down the easiest I can. This issue seemes to be developing with increasing cases as the age of these cars increase. The most popular years are 2007-2008 Legacy GT, 2009's I suspect could have this issue although I have not confirmed whether or not subaru revised the gasket for 09.
The issue presents itself in a rough idle when the outside temps are at or below freezing. This seems to only happen during the first start of the day, or with an extended cooling period in these temps (cold starting). The engine will initially idle fine up until the blue light (sport sharp delay) shuts off. Once the engine has semi-warmed the rough idle will appear. This can cause stalling when the car is stopped, and obviously a rough idle. The rough idle then disappears once the engine has reached normal operating temp.
This is caused by the orange o-ring type gaskets that sit between the plastic intake manifold and the tumbler valves (TGV) that have constricted due to cold temps. They shrink to the point of causing a gap where air will flow into the TGV and into the combustion chamber causing non-metered air to aid in combustion, since it's non-metered, the ECU will not add fuel to accompany that air, therefore the engine will run lean and misfire. Once the TGV aluminum warms from the engine heat, the gaskets soften and seal the area, causing an air tight seal again eliminating the gap and resuming smooth idle and operation.
The check engine codes related to this particular issue shows itself in a System too lean, misfire cyl 1, 2, 3, 4 or a combination of these misfires.
The part number for the new o-rings is #14035AA492, you will need four of them. You can get these at the dealer for around $20 total.
These gaskets are revised from the OEM ones, they have a larger surface contact area.
We will start by removing surrounding items and bolts. The yellow circles and arrows represent a need for removal or loosening. The intercooler needs to be completely removed, arrows are showing the bolts needing to be removed in order to pull it off with the BOV attached to it. The intake elbow will need to be pulled off as well in order to get at the manifold in a full view.
Once these items have been removed, we can begin to tackle the manifold itself. We need to remove the wiring harness plastic bracket that attaches to the intercooler side of the manifold(in pic below). This harness makes moving the manifold around difficult, so we will part it from the manifold to aid in maneuverability. Use your own discretion to loosen any items you notice that is holding the manifold down, such as sensor clips, bolts to various other engine bay items, etc... Now remove the turbo coolant resevoir, unbolt it from the manifold and disconnect all the hoses from it.
Next we can get to the actual manifold bolts, these are circled below, six on each side. Remove these. You'll now be able to move the manfiold around. NOTE: fuel lines do not need to be adjusted on the drivers side of the manifold, these pass through the center of the plenums(seen in pic 1)
Once the manifold bolts are removed, check to see how far you can lift the manifold upwards, all you need is about an inch space between the manifold and TGV's. It's easiest having someone hold the manifold up while you use needlenose pliers to grab the tabs of the old o-rings(seen in pic 3). Pull old o-rings out and install new ones, NOTE: the o-rings sit in the manifold, you'll notice this in pic 3. Once you placed the new o-rings in, confirm they are seated in the manifold, you can do this using a handheld mirror or by sweeping an L allen key along the base of the manifold. You will have less lifting room on the driver's side due to the fuel lines, but it is still enough to swap out the o-rings.
pic shows a different manifold however the gaskets are seated exactly like ours.
Once you've installed the new o-rings, and made sure the gaskets are seated, you can start reinstalling the six manifold bolts. These are to be torqued at only 6ft-lbs per subaru service manual.
Reinstall all removed items and loosened clips/hoses etc...and you're good to go!