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Old 11-06-2020, 10:56 AM   #1
Scooby Newbie
Member#: 520053
Join Date: Nov 2020
Default 06 Impreza 2.5 AT low idle issues

I'm ripping my hair out trying to find the cause for the car idling at 250 rpm or so while in gear.

I've cleaned the throttle body, MAF, replaced the upstream 02 sensor, reset the ecu and idle relearning procedure. Checked for vacuum leaks, exhaust leaks, changed the fuel filter. Spark plugs were changed about 10k miles ago.

Other than the low idle, the car runs just fine. Accelerates at partial and WOT fine, no missing or power loss. When I come to a stop the car dips down to 250rpm. I had one CEL p0171 at the start of the issue but it has not come back yet.

Any advise on where to look next?
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Old 11-06-2020, 11:06 AM   #2
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Member#: 118334
Join Date: Jun 2006
Chapter/Region: SWIC
2006 Red GDA-F
2005 Aspen White GDA-E


Does the idle increase properly when AC is switched on?
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Old 11-06-2020, 11:30 AM   #3
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The idle stays the same while in gear and at idle with the AC on as well as the defrosters. I can hear engine take a bit of load but no fluctuation in the RPM.

bad AC clutch?
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Old 11-07-2020, 10:12 AM   #4
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Join Date: Apr 2016
Chapter/Region: Tri-State
Location: Rafferty Subaru, PA
2002 Outback Sport


You had a P0171, so I'd start with that. I know the light isn't on now, but you can have a lean condition that's enough to cause symptoms, but not enough to turn the CEL on.

A vacuum leak would cause a low idle for sure. If you have access to a scan tool that shows live data, you can check the fuel trims at idle- anything over 10% can cause symptoms. The code doesn't usually set until 18%-20% or so.
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Old 11-09-2020, 06:07 PM   #5
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Member#: 348982
Join Date: Mar 2013
Chapter/Region: NESIC
Location: Shoreham, VT
2002 TS


Also the break booster could be going bad maybe? With the vacuum pressure en such could be another place to look. I would also check to make sure all the intake parts are tight. The fuel pressure reg could have gone, very unlikely but maybe not enough fuel pressure even though your saying it is fine on WOT. I'm just spit balling ideas for ya
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Old 12-04-2020, 11:35 AM   #6
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Join Date: Jan 2009
Chapter/Region: Tri-State
Location: /r/subaru, NJ
i been hatin'
Hellas since 2012


Could it be a loose throttle cable? A question of tightening the nuts or is it drive by wire?
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Old 12-06-2020, 09:40 PM   #7
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Join Date: Feb 2007
Chapter/Region: E. Canada
Location: Quebec, QC
2019 Crosstrek Sport


Just to make sure - how do you know it idles at 250 rpm since the tachometer starts at 500?
A 250 idle would be rattling as hell, not to mention probably triggering the battery light due to insufficient alternator output voltage.
Just checking!
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Old 01-16-2021, 10:27 AM   #8
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Pupwagn, did you solve the problem? I am having the same problem although manual car. No codes. Also did all the things you mention here except O2 and fuel filter. Also, I have some hardly noticeable misfire (popping exhaust) at idle when hot. But for me, idle climbs slightly when AC is on. Also when driving at very low speeds and very soft throttle the car is "jumpy", I think it's related to the grumpy idle.
When I start the car (cold or warm) I have noticed that it sounds "heavy" in a way. You don't have to crank it for long but it sounds somewhat sluggish and doesn't fire up as strong and eagerly as my 3.0 H6 does (this is my first 4 banger Subaru so I don't know what to expect really). Battery holds charge well so I don't think it's that simple. Maybe bad ground somewhere?

I have access to a computer to log but I don't know what to look for.
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