Rough Idle ??? - Subaru Outback - Subaru Outback Forums

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Old 09-21-2009, 08:46 PM   #1 (permalink)
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Unhappy Rough Idle ???

Just wondering what the cause of a rough idle only when the Outback is in gear and at a stop.

When the engine is cold (RPM's around 1,000) theres no idle problem when I come to a stop light/sign.

After the engine has been running for about 5 min or so (RPM's roughly 600 to 800)and I come to a stop light/sign it will start idleing rough but will never stall out.

What's the deal here?!?!?!


P.S. It's a 2005 Outback XTL 5-speed Automatic 84,000 miles.
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Old 09-22-2009, 03:57 AM   #2 (permalink)
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When was the last tuneup ?

I can tell mine needs a tuneup when it starts to idle crappy.

Cheap things to do: Clean the IAC, Throttle body, PCV,

Check Air filter, Fuel filter, Use some injector cleaner, Check the plugs, Wires, Boots for oil contamination.

Under load at low rpms is when the poor state of tune show the worse.
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Old 09-22-2009, 10:58 AM   #3 (permalink)
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I recently (2 months ago) had the spark plugs & wires changed oil and filter and the air and fuel filter. The dealership also added some fuel injector cleaner. All of this improved the idle problem a little but it's still not smooth.

Looks like next I need to look at the IAC, throttle body and pcv. Thanks for the reply!!!

Any idea where the IAC is located on a 05 outback?
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Old 09-23-2009, 05:19 AM   #4 (permalink)
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Should have looked at the book first.

Apparently on the 04 and up turbo models and the 05 and up N/A models, the IAC function is part of the Drive By Wire system and the ECM controls the throttle plate angle to meter air during idle.

I have no experience with the drive by wire on these cars. On our BMW which has a similar system, I clean the throttle body out really well and make sure the area where the throttle plate seats in the body doesn't have a carbon build up there. The throttle plate won't be open very far during idle so if theres gunk in there, it will have a pretty big effect.

If that doesn't help, there may be a way to adjust the throttle plate angle or possibly Zero the throttle position sensor [need to go look at the book again to see if that's also part of the drive by wire system]

Let us know what you come up with, maybe someone with an 05 will chime in or ask in the turbo section.
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Old 09-23-2009, 03:01 PM   #5 (permalink)
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Think I'm going to have the throttle body cleaned first. Is this something I can do myself (not very good with mechanics) or should I take it to a shop and have them clean and inspect it?
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Old 09-23-2009, 09:03 PM   #6 (permalink)
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possible timing belt ?
I know 87k is a little low, but its not out of the question, and that is a good indicator of a worn timing belt, a rough idle.
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Old 09-24-2009, 12:51 AM   #7 (permalink)
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hmmm...timing belt could be a possibility, but after the engine has had time to warm up (35-45 min) it seems to idle the way it should.
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Old 09-28-2009, 08:02 PM   #8 (permalink)
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PCV valve replaced. That didn't seem to help the problem.
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Old 10-19-2009, 09:33 PM   #9 (permalink)
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Default Tranny Fluid

I've been reading a lot that the tranny fluid might need to be changed. I have the same problem right now and I was planning on changing my tranny fluid on wednesday... so I'll let you know if that did anything.

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Old 10-19-2009, 10:10 PM   #10 (permalink)
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Mine does the same thing. But only sometimes it gets better. Ive noticed that certain fuel makes it worse. Its also worse when its really really cold.

Ive read on other forums and from manufacturer TSB's that the PZEV systems can cause a slightly rough idle. The high ethenol content of the fuel makes it worse. Temperatures below 50 degrees F also. The PZEV system is a lean burn system. All 3 factors contribute to my rough idle. I can tell the lean burn is a major factor when I drive next to a wall at low speeds/high load. The sound of pinging is very evident.

Maybe my knock sensor is bad?

I also thought about reprogramming my fuel map to richen it slightly or adjust the idle speed.
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Yeah, mine shakes at idle, they all do that..........right?
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