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2006 Subaru Outback rough idle and stalling

I have a manual 2006 subaru outback 2.5L XT that idles rough pretty much all the time and the rpm gauge fluctuates at stop lights between 500-1000rpm. The car also stalls at low speeds and almost always about 1/2 a mile from my house when the engine is cool and at a stop sign or light. It seems to not do it as much when the engine is warm. No mechanic can figure out what it is yet. Gas mileage is getting bad because of it, check engine light is on. One mechanic thought then an engine coil because there was a misfire in cylinder 4. He switched that one to 3 to see if it would throw the check engine again and it did not. Then they thought it was the fuel injector because they were switched and now they are back to normal. Now they think it is a head gasket. Any help?


  • edited December 2010
    Have they checked the valve lash? Valves out of adjustment can cause symptoms like this.
  • edited December 2010
    No no one has mentioned this yet. I finally to it to the dealer to have it checked. I have read about several things that can cause this. Some include: ECM/ECU Reflash, MAF sensor, Idle Air Control valve, Vacuum leak, Fuel Pressure problem. Do any of these sound like they are of valuable consideration? We will see what the dealer finds tomorrow. I will post with his diagnostic findings.
  • edited December 2010
    Since this problem could be major or minor, the first thing I would suggest is putting a vacuum gauge on it. This is easy to do, will reveal much, and unfortunately is a tool that many mechanics do not use.

    If the manifold vacuum is not correct then a compression test should be run. If the compression is low on any cylinder then the valve lash should be inspected. If the valve lash is tight (especially with an exhaust valve) adjusting the lash may be a short term fix because a tight valve is often damaged. It will only get worse over time.

    A vacuum gauge could also reveal whether or not there's a vacuum leak. A vac. leak can cause this kind of symptom and an Idle Air Control valve problem can do the same.
    From the sound of things these mechanics are skipping basic diagnostics and throwing parts at it.
  • edited December 2010
    So what I am hearing from the mechanic right now for an initial diagnostic that the issue has to do with the turbo and a bad o-ring. They have had a couple turbos in this week that have had the same issue with the same symptoms: running rough when cool, stalling, and running a bit smoother when the engine is warm. I made mention of the vacuum gauge to the person I talked to, but he seemed the think the mechanic was certain that this was the problem. Any thoughts?
  • edited December 2010
    just to clarify-they think it is a bad intake o-ring...
  • edited December 2010
    Slightly skeptical. The turbocharger is pretty much irrelevant at idle speeds and the only way an O-ring would cause an idle problem would be if it allowed an air leak that was below the throttle plate, not in the intake tract between the throttle plate and turbocharger.

    Any kind of minor air leak above the throttle plate should not affect the idle but could possibly cause a hesitation or bucking problem.

    It would be interesting to know which O-ring he's talking about and where it's located.
  • edited December 2010
    So the mechanic at the Subaru dealer came in with a final diagnostic. I am told there is a service bulletin out on this issue from Subaru how to fix this if there are symptoms similar to mine. The seals @ the intake manifold need to be replaced. The mechanic did a compression test on the cylinders that were throwing the check engine light and those valves checked out fine so he was able to eliminate that. I guess there have been a couple cars in this week at that shop with this problem, 3 out of the 4 were turbos. I am also told that the issues arise the colder the weather gets. He said he has seen quite a few 2005's do this and one 2008. Mine is the first 2006 he has seen. They say that they stand by their diagnostic, so we will see if that solves the problem. Any final thoughts? I will let everyone know how it turns out.
  • edited December 2010
    An intake manifold leak below the throttle plate will cause a rough idle on any car made and will often be more noticeable when cold.
    Again, spending 10 seconds connnecting a vacuum gauge would show this instantly, just as it would any problem involving tight valves and compression.

    Glad the compression test came out fine although the numbers are not given here. Ideally, you're looking for 180-190 PSI on each cylinder.
  • edited December 2010
    I will find out the numbers. So what is the difference or how to you tell if it is the seals at the intake manifold or the head gasket. Is there a difference?
  • edited December 2010
    Any leak at the intake manifold will not show up on a compression test. If there is a head gasket problem that caused a breach into one or more of the combustion chambers then yes, that will show up.

    While it varies a bit, the numbers you're looking for on a compression check should be in the 175, 180, 190 PSI range and all numbers should be fairly close to each other.
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