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Old 05-13-2018, 04:01 PM   #1
alanlcit
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Member#: 71108
Join Date: Sep 2004
Chapter/Region: SCIC
Location: San Diego, CA
Vehicle:
2005 WRX STi
Silver

Default Pulsing/Oscillating Idle & Acceleration when NOT Wide Open Throttle - 2005 STi

Problem just started yesterday, car has been fine until now:

2005 STi
146,000 miles, stock except Cobb high flow downpipe & AP tune.

- Idle speed is unsteady (oscillates between 750 and 1000 with about ~2 second period). Very regular oscillation period; it's not occasionally trying to stall and then recovering. Both idle speeds are healthy and smooth enough.
- When driving and applying accelerator modestly (to maintain speed, not to accelerate), car will surge forward slightly then settle, then repeat, continuously, with an extremely regular ~2 second period. Impossible to maintain perfectly constant speed (there is very small acceleration, then deceleration - maybe 1-2 mph oscillation in speed, with ~2 second period)
- With Wide open throttle, seems fine, power seems to all be there and as long as throttle is wide open, no hesitation or surging/relaxing is felt.
- No CEL. Haven't hooked up to read any pending codes yet.
- The oscillation is such that the boost/vacuum oscillates by about 1 or 2 PSI on the gauge, whether there is a little bit of vacuum or a little bit of boost, it doesn't matter.

I'm assuming it can't be MAF since that is used open loop. It's not that dirty anyway; I cleaned it a couple years (5000 miles) ago.

I looked for blown vacuum hoses. Last time I drove it without problems I did drive it hard briefly before getting home, so I figured something might have blown off. I don't drive it often these days, so it's been sitting for a week or so.

Looking around at other threads, it looks like the front AFR sensor is the most likely culprit. It does have 146,000 miles after all.

If anyone has any other ideas, or comments, let me know. I have heard TPS as a possibility but it seems like that would have subtly different symptoms, not so regular. I would also think that such a problem would depend on the throttle position (this issues does NOT depend on throttle position, as long as you aren't wide open).

I'm planning to just buy a new AFR, replace, and see whether the problem goes away.
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Old 05-13-2018, 09:47 PM   #2
alanlcit
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Member#: 71108
Join Date: Sep 2004
Chapter/Region: SCIC
Location: San Diego, CA
Vehicle:
2005 WRX STi
Silver

Default

Ordered a new Denso 234-9120 AFR/upstream O2 sensor today (looks like that is equivalent to the OEM sensor part # 22641AA211).

Also experimented with unplugging the front O2/AFR sensor. That made the oscillation go away at idle, but of course turned on the Check Engine light. I don't think that is definitive proof that it is the front O2/AFR sensor, but seems like a replacement at 146,000 isn't unexpected.

Removed the plastic splash guard piece from the wheel well which looks like it is there for the purpose of accessing the sensor for removal, so hopefully I'll be able to remove it and reroute the wire without having to remove anything else. I'm hoping to be able to undo the plastic push in clips from the metal holder (which is attached via the passenger rear coil pack bolt) without having to actually remove all the stuff I'd have to remove to get easy access for coil pack removal. I'll give it 5 minutes of trying before I give up.

Hopefully the sensor comes out easily with the sensor removal tool...

I'll update with the final result for the record. Hopefully it is the final result, anyway.

Last edited by alanlcit; 05-13-2018 at 09:52 PM.
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Old 05-16-2018, 02:47 AM   #3
alanlcit
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Member#: 71108
Join Date: Sep 2004
Chapter/Region: SCIC
Location: San Diego, CA
Vehicle:
2005 WRX STi
Silver

Default Installed AFR, all is well!

Installed the new AFR, and now the idle is rock solid and the acceleration/deceleration oscillation when cruising is gone. Check engine light which appeared when I unplugged the old sensor also disappeared after a couple start/stop cycles.

Had to remove the air filter box to unclip the little plastic bits, and had to replace a ziptie to secure some wires together, but everything went as expected.

I did have to use an offset O2 sensor removal tool. The straight on one didn't have enough clearance to the steel portion of the wheel well.

Fixed!
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Old 05-16-2018, 09:18 AM   #4
Charlie-III
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Join Date: Dec 2002
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Location: USA, North NJ, 07456
Vehicle:
1998 Legacy 2.5GT
Silver Sleeper BK, 5MT

Default

Glad you got it fixed.
Thanks for the followup, I hate dead end threads.
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Old 05-16-2018, 02:02 PM   #5
alanlcit
Scooby Newbie
 
Member#: 71108
Join Date: Sep 2004
Chapter/Region: SCIC
Location: San Diego, CA
Vehicle:
2005 WRX STi
Silver

Default Addendum

Charlie,
No problem.

Just a quick addendum, following up on the basic details for a Subaru WRX STi 2005 upstream O2/AFR replacement. More or less this is all covered in the FSM:

Denso 234-9120, Subaru 22641AA211

1) You don't need to remove the bottom plastic cover from the engine. You don't need to remove the heat shield from the exhaust manifold.
2) You just need to remove 4 pop-its from the plastic cover in the wheel well, and the sensor is accessible.
3) A short offset O2 sensor removal tool is a must, plus a 6-inch extension, to loosen the sensor.
4) Tightening torque for sensor is 30N-m, 22.1 ft-lbs.
5) Anti-seize is recommended by the service manual ONLY on the threads. My sensor came with anti-seize pre-installed on the threads and I didn't add any more.
6) It's probably easiest to bite the bullet and just remove the air cleaner box. It's just two bolts, but it's recommended that you have at least 12" of extensions to easily access them (then it's easy!). Tightening torque for these bolts is 33Nm/24.6ft-lbs. Removing the air cleaner box allows easy access to the plastic clips for removal and replacement, which hold the sensor wire in position. If you are very gifted you might be able to get away without doing this.
7) There is a single zip tie to be replaced where the sensor cable comes by the power steering return hose and a couple other wires/vacuum hoses. Might be able to get the connector through without cutting it, but it's just a simple black zip tie...I threaded the new one through before cutting the old one, since the zip tie actually holds another fastening clip in position...
8) Make sure the cable is clear of the turbo housing. It should be no problem when it is properly secured.
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