DIY: Rough idle in cold start/cold temps LGT 07-09

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.




NOTE: 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!

sweet, thanks for the write up! how long did it take you about?

About an hour....

Many-many thanks for posting this.
I have an 08'LGT and have recently gone to a Cobb stage 2 tune with a more free flowing air-filter and colder plugs( other supporting mods for the tune). I was out the other night and experienced the noted rough idle. I cursed the mods and thought about all the things it could be. It was also -5 out (if you haven't guessed, I live in Chicago).

I'll be getting some gaskets this weekend and thanks for the 'how to' above.

Power to BMX045( I grew up in Arlington MA- word to Btown)

no prob chicago, lemme know if you have any questions...

well crap, I thought it was my plugs, but now that I think of it, it hasn't really been that cold since I did the plugs... so looks like I'm getting some new o rings. Thanks for the info.

Originally Posted by pieboy68

well crap, I thought it was my plugs, but now that I think of it, it hasn't really been that cold since I did the plugs... so looks like I'm getting some new o rings. Thanks for the info.

Was hoping for that too. Just picked up my spec. B with 22k miles on it and noticed a rough idle as described. Hopefully the new revised gaskets fix the issue for a while. I'd hate to do this again next winter. Any chance this could be covered under warranty or with a TSB?

Originally Posted by murdoc158

Any chance this could be covered under warranty or with a TSB?

I will be finding out, hadn't thought of that.

Just finished this. Will know tomorrow morning if it did the trick. Gaskets were $5.25/each at my local dealer. The walk-thru was very helpful, Thanks.

The clearance on the drivers side because of the fuel lines sure did make things difficult. And, the the intake manifold bolt closest to the turbo took me a while to figure out how to get to it (ended up using a screwdriver to pry just enough space between a hose and the manifold to get the socket through).

Aside from, that it went well. Took us about 2 hours.

Update (Day 1): Was 39º out this morning, which would is low enough to get the rough idle - Zero stumbling and overall the idle felt much better.
Update (Day 2): Was 37º this morning. Idle was smooth. : )

I bring the car in with this page print out. They decide to change all the gaskets which is the following parts:
14035AA421 x1
14035AA492 x4
16175AA331 x1
14497AA010 x1

I hope they know what they are doing

Picked up new gaskets for $17 (They told me 5.23 a piece over the phone as well as an extra shipping charge, but that wasn't what they charged me).

Took nearly 3 hrs altogether, this walkthrough was very helpful. Here is what I have to offer:

If you have some dental picks it makes getting the old gaskets out a breeze. I ripped one of the tabs off one of them with needle nose, so the pick was a must. Some plugs to stop off the coolant reservoir and lines make things less messy. I actually didn't completely remove the reservoir. I left the bottom line attached and had plenty of play to get the pass side done.

From the picture, the blue arrows show additional bolts/plugs that I left in place and everything worked just fine. I did, however, remove the air pump (circled, three bolts), which gave lots more room to work with the front gasket on the driver's side. Also took off the bolt (orange arrow) that holds the fuel lines down to gain some extra play.

Just take time while seating the new ones to make sure you don't have a fold or something crazy when you bolt the manifold back down. A small mirror is super helpful.

The weather should get cold enough again by the end of the week for me to tell if it did the trick. Good luck!

*Updadte: Survived all through last winter ('11) and still doing fine this winter although I am out of the cold of MT.

Hi, I'm a new member. My car is acting up. All I did was disconnect the negative terminal on the battery. Den I reconnect it. And now the idle is low and the car dies out.

Since you are in Hawaii, I doubt you are having this same issue. Are you getting a CEL?

No CEL. I think its just the ecu or the map.

Just had these seals replaced on my 09 Outback XT under warranty. I had to take thar car back 3 times for them to figure it out. I told them the 1st day I took it I assumed it was this. Kept throwing the same code: lean cylinder bank.
They had the car about a week while a new MAF was being ordered. Called to say the car is working great. Drive it home 4 miles and the light comes on and started running like crap. Turn it around and took it straight back. Oh, it must be the sensors in the exhaust. It will take us a week because the part need to be ordered. The same evening after picking it up that 2nd time it started again.
They then blamed it on bad gas. So I drove it a few days till the tank was empy and filled it up at a different station. Still ran like crap. Took it in the 3rd time and repeated my opinion of the seals at the intake manifold. Oh no, he said, they checked all that. Two days later he calls to say we found the problem! The seals are letting in air after it is metered. Well Duh...
Anyways, it was done in 2 days the 3rd time and the car is idling smoothly now. Amazing how it took 3 times and 3 weeks to get it right.
I'm done with this car and maybe Subaru. Blown engine at Christmas and now this not even 3 months later...
An engine rod started coming loose with 19,000 on the clock Christmas Day. Took 2 weeks for them to rebuild the engine. I really think I got a bad car.