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2008 Subaru Outback 2.5i
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Discussion Starter #1 (Edited)
Hello all,
I’m new to the outback group, having just bought my 2008 Outback 2.5I a few months ago. After purchasing the car I fixed the P0134 (or p0130, I can’t remember...) OBD code with a new Upstream O2 sensor. The problem went away.

Recently, I started to notice a lack of power from the engine, especially when climbing hills and during acceleration. After having to almost floor the accelerator to merge up a hill onto the highway, an “oxygen sensor slow response” code was thrown. I drove the car a few more days after that, and it still felt fine, if a little down on power.

Today however, I drove into town, and on the way back, the car refused to accelerate almost at all. The engine would rev up and the car started a rhythmic cycle of accelerating for a second, then bogging down and slowing. It got the the point that I couldn’t even reach 25 mph on the mostly level road home. After this, I replaced the downstream O2 sensor and cleared the OBD codes, however the problem persisted. During a 10 minute test drive, no OBD codes were thrown, and the problem seemed to get worse as I drove. When in neutral, the engine had difficulty reving past 3000 rpm and the rpms would pulse and fluctuate while the accelerator was held steady.

Thankfully I live next door to a mechanic shop, so I plan on dropping it off on Monday when they reopen. In the meantime, I’m thinking it may be a fuel issue, like a clogged filter or bad pump. Does anyone have any other ideas as to what could cause this so suddenly?
Also, any advice for diagnosing the fuel system?

Thanks!
 

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On the Super Mod Squad
2002 Pair: 3.0 VDC Wag & 2.5 Limited Sedan
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26,193 Posts
what brand o2 sensors did you buy?

subarus like Denso and OEM. others may cause problems. (including data quality disagreements between the front / rear o2 sensors. )

@plain OM any suggestions.
 

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2008 Subaru Outback 2.5i
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Discussion Starter #3
what brand o2 sensors did you buy?
The Upstream was replaced with a Denso

I replaced the downstream with a Bosch after the issue began.
 

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Have a laptop? Download and set up the Romraider scanner program (free RomRaider - Open Source ECU Tools | RomRaider / RomRaider) and get an inexpensive Vag-Com KKL 401.9 OBD-to-USB adapter cable. Then use to monitor and log the data from the ECM related to sensors and controls. The data might well identify the problem and point to a cause.

Ignition, fuel supply, fuel control, or intake and exhaust restrictions could also be factors. But I wouldn't suggest replacing components in the hope one or more of them fixes the problem. Many faults can be identified using the ECM data stream -- that's what it's for.

The shop next door probably will use a scanner to help identify the problem.
 

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2001 VDC/SC One of a Kind
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No misfire codes? Those would be P0300 thru P0304.

I can think of a few things. You can knock them off the list if you know it's good, or it's been checked.

Battery - has to be supplying the amperes it's rated at or higher when checked cold. If it's an Auto Zone or Walmart battery, that may well be your problem.

Grounds - all of them need to be clean, tight and free of corrosion. Using a multi-meter to check resistance, the highest on the main ground cable from post to lug by the starter is 2 ohms. The engine grounds the same. Body grounds can be up to 4. No more.

Positive battery cable and alternator cable - these are the same measure as the grounds. Alt to fuse box, post to fuse box, and you may as well check the post to starter.

Next is how many fill ups have you had since the car started acting up? Where did you get the fuel right before it started? There was a problem here in Austin, for example, where the delivery driver pumped diesel in to the regular unleaded tank. Lots of cars got F'd up. Not saying you got diesel, any thing is possible though.

Your car uses the front and rear O2 data to adjust fuel trim. That rear sensor needs to be a Denso. (Denso is the OEM sensor.) The Bosch is too slow.

Fuel flow and pressure is checked at the fuel rail where the fuel hose connects to the rail. The fuel filter is in the tank with the fuel pump. It's in the plastic cylinder at the top of the module. This could cause the engine to run lean, lose power, stall, or, a combination. It can cause a lean code or AF code to pop up. Whomever checks the data needs to first look at the fuel trim, check for vacuum leaks and verify power supply. DO NOT let them change that MAF sensor.

Your car is drive by wire (DBW). Is the throttle clean?

When was the timing belt and components replaced last? Do you hear any tapping/knocking from the front of the engine, driver side?

If you trust this shop with your Subie, it's all good. If they tell you something oddball that seems out of line, post back up what they say they find.
 

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2001 VDC/SC One of a Kind
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I thought I put it in there, but also check that the alternator is working. The only way to accurately test the alternator is with a good battery and cabling. If the battery and/or the grounds are bad, or the positive cable from the alternator to the fuse box is bad, the alternator won't work right.
 

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2008 Subaru Outback 2.5i
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Discussion Starter #7
Thank you all for replying so quickly!
You guys are the best!
A few quick responses:
Where did you get the fuel right before it started?
The Same place as always, and the problem only really got bad once I was at 1/3 of a tank.

When was the timing belt and components replaced last?
I don't know about that. The previous owner didn't really like putting a lot of money into the car (Source: The $13 oxygen sensor that I replaced)
I've taken a look at it by removing the right cover, however I have not had time to perform a through inspection.

check that the alternator is working.
I believe it is, as my OBDII diagnostic tool was reading 14 volts during yesterday's test drive, as opposed to the 12.5 volt cold (23°F) reading.

Also, During my test runs yesterday, both the old cheap downstream oxygen sensor and the new one I installed were both reading at 0.9 volts, which is very rich. Didn't look at the trim though...

No misfire codes? Those would be P0300 thru P0304.
I rescanned, and it had a pending P0302 (Cylinder 2 Misfire) code this morning when I moved it into the garage.
 

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Also, During my test runs yesterday, both the old cheap downstream oxygen sensor and the new one I installed were both reading at 0.9 volts, which is very rich. Didn't look at the trim though...
What did your test runs consist of, and were both sensors reading 0.9 V all the time?

The Rear O2 Sensor voltage can be at 0.9 V at idle or steady-state cruising, but should drop to zero when the throttle is released while cruising. It will also vary with changes in the throttle, but stabilize in the 0.6 - 0.9 V range when the throttle, engine and vehicle speed is steady.

If that OBD diagnostic tool is using the ECM's OBD data, as far as I know the (front) A/F Sensor voltage should be closer to 2.7 V. The 0.9 V seems suspicious. Are you sure that's what it was reading for that sensor (B1S1)?

Incidentally, the 2008 2.5 non-turbo does not use the P0130 code, but it does use the P0134, which is (italics added):

DTC P0134 O2 SENSOR CIRCUIT NO ACTIVITY DETECTED
(BANK 1 SENSOR 1)
 

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2008 Subaru Outback 2.5i
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Discussion Starter #9
If that OBD diagnostic tool is using the ECM's OBD data, as far as I know the (front) A/F Sensor voltage should be closer to 2.7 V. The 0.9 V seems suspicious. Are you sure that's what it was reading for that sensor (B1S1)?
I should probably clarify.
The 2 sensors I am referring to are both the downstream one. The upstream sensor was replaced a month before.
I test drove it once with the cheap downstream sensor that the old owner had installed. Readings were around 0.8 to 0.95
I then replaced it (The downstream sensor) with a new Bosch one. Readings stayed around the same.
During both tests the Denso Upstream sensor stayed around 2.6-2.8 volts (Normal range)
 

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2008 JDM Outback 3.0R, 5EAT
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Today however, I drove into town, and on the way back, the car refused to accelerate almost at all. The engine would rev up and the car started a rhythmic cycle of accelerating for a second, then bogging down and slowing. It got the the point that I couldn’t even reach 25 mph on the mostly level road home. After this, I replaced the downstream O2 sensor and cleared the OBD codes, however the problem persisted. During a 10 minute test drive, no OBD codes were thrown, and the problem seemed to get worse as I drove. When in neutral, the engine had difficulty reving past 3000 rpm and the rpms would pulse and fluctuate while the accelerator was held steady.
A friend had similar symptoms on his 2006 2.5, turned out to be the catalytic converter. Can you log the MAF sensor reading whilst accelerating ? That would show if the airflow is restricted, @plain OM helped us diagnose the nominal airflow flow on this thread .
 

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Can you log the MAF sensor reading whilst accelerating ? . . . turned out to be the catalytic converter.
Thanks for that. The symptoms do appear similar, and that was one of the reasons for suggesting Romraider logs back in post #4. Of course there's other possibilities, as @cardoc mentioned. With these drive-by-wire, computer controlled engines, there's multiple feedback loops that alter the way symptoms appear; it's harder to diagnose based on simple descriptions. Probably why they set up the ECM not only to control the engine, but to output a wide range of data that shows what it's doing and where the ECM is introducing corrections due to odd running behaviors.

If the car goes into the neighboring shop tomorrow, they will have the advantage of experiencing the symptoms, and probably the opportunity to monitor the data streams using a professional scanner.

Hope @Jepherson will keep us informed . . .
 

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2008 Subaru Outback 2.5i
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Discussion Starter #13
Can you log the MAF sensor reading whilst accelerating ?
I would, however the problem has gotten so bad the car can't even break 15 mph in my level driveway anymore. "Acceleration" is almost non-existent, so I don't want to take it out on the road unless it's to take it next door to the mechanic.
I tried to get a fuel pressure gauge to test the fuel system, however my local store didn't have the right type.
I plan on dropping it off tomorrow afternoon at the shop.
I'll keep you all updated on their findings.
Thank you so much for helping me out!
 

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2001 VDC/SC One of a Kind
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The misfire is related. So this leads to AVLS operation, spark plugs, coil, and back to grounding. The timing belt can also still be an issue.
 

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Discussion Starter #15
A friend had similar symptoms on his 2006 2.5, turned out to be the catalytic converter.
Yep, got word back from the shop. The cat was smooth as a pipe and everything downstream was clogged up. I had a feeling, as I had been having some cat issues before the power loss. Guess I didn't need the new downstream oxygen sensor. oh well...
I'll be getting new cats tomorrow if all goes to plan. I'll keep you updated on the result.
Thank you all for helping out!
 
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