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Old 05-27-2015, 10:41 PM   #1
kumquat
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Member#: 412821
Join Date: Feb 2015
Location: NY Capital Region
Vehicle:
2009 Outback Sport

Default P0172 (rich), low RPMs/chugging at idle, fine with open throttle

I have an 09 Outback Sport. Today, the car somewhat suddenly developed a problem. The RPMs drop (and sometimes surge a little) while sitting at idle, and while it hasn't actually stalled out it sometimes almost does. Worse, when I try to take off from a stop, there is zero throttle response until I push the pedal in pretty far.. the RPMs actually drop a little when I first open the throttle a little.

Once the car is moving, everything is fine.

The CEL hasn't come on yet, but my Ultragauge says a P0172 is pending.

When I got home, I looked at some of the values on the UG. At hot idle, in Park, short term fuel trim was -8, long term fuel trim -22.7. Manifold absolute pressure 3.92 psi (7.98 inHg); Bank 1 O2 0.69 V, engine load 24.7%, absolute throttle position 30.6%, relative throttle position 2.35%.

Earlier while cruising on the highway, STFT was -25 and LTFT -13.3, with MAP 9.14 psi at 80% load.

I haven't done much to the car lately. Oil change and rear diff fluid on Saturday, and I took a look at the air filter, which seemed OK. I have some new NGK iridium spark plugs I plan to put in.

What could this be? A vacuum leak? Where should I start looking?
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Old 05-27-2015, 10:55 PM   #2
skramguts
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Member#: 379316
Join Date: Jan 2014
Chapter/Region: SCIC
Vehicle:
2006 Legacy BSM
2009 Impreza DGM (RIP)

Default

Sounds like your classic case of O2 sensor bank 1 (front) failure. If you do a bit of research others have the problem you're mentioning. Also try to do some MAF cleaner on the intake sensor
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Old 05-27-2015, 10:57 PM   #3
kumquat
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Member#: 412821
Join Date: Feb 2015
Location: NY Capital Region
Vehicle:
2009 Outback Sport

Default

Thanks, I was going to get some MAF cleaner but the parts store was closed. I'll probably try that tomorrow.

Is there anything I can look at via the scan tool or otherwise to narrow it down to the O2 sensor vs something else? I'd really like to avoid spending $100 if I could.
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Old Yesterday, 02:54 PM   #4
kumquat
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Member#: 412821
Join Date: Feb 2015
Location: NY Capital Region
Vehicle:
2009 Outback Sport

Default

I dropped in a new air filter and hit the MAF with some cleaner this afternoon. I also put some Seafoam in the tank. I reset the code afterward.

These seem to have helped a good bit, but it's still not quite 100%. The RPMs tend to drop to 400-500 briefly when coming to a stop after moving, before picking back up and settling at 750-850. At hot idle in Park, STFT is 0.78% and LTFT is -3.9% which seems much improved. I didn't catch either fuel trim dropping down to big negative numbers (like -29%) over about 20 miles or so of city and highway driving.

I'll be replacing the spark plugs this weekend and I'll disconnect the battery for a while to make sure the computer is fully reset, then I'll wait for a while and see how it is.
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Old Today, 04:59 PM   #5
skramguts
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Member#: 379316
Join Date: Jan 2014
Chapter/Region: SCIC
Vehicle:
2006 Legacy BSM
2009 Impreza DGM (RIP)

Default

Good idea to replace spark plugs. May want to spend the extra cash to change the wires as well. Remember to use dielectric grease on the contacts when you're changing plugs or wires, helps prevent corrosion. You're going to want to use NGK plugs as well. I used NGK plugs and wires and haven't had a misfire since replacing wires.
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Old Today, 05:00 PM   #6
skramguts
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Member#: 379316
Join Date: Jan 2014
Chapter/Region: SCIC
Vehicle:
2006 Legacy BSM
2009 Impreza DGM (RIP)

Default

Oh if all else fails then bite the bullet and do the O2 sensor, those last 100k miles (usually)
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